Teemo shrooms can do tons of damage for cheap (especially with liandires) and last for 10 minutes from the get go, and also have a relatively low cooldown.

With his shrooms Teemo can dominate vision and allow his team to secure objectives, make picks, and limit downside risks - and in the long run scare his enemies from even trying to enter large swaths of the map without 1) getting picked because Teemo's teams been aware of his movements since he entered the area, 2) getting burned for a decent chunk of damage by popping his shroom; often both will happen at the same time.

In soloQ, particularly low elo soloQ, because he is such a rare pick as a support (usually picked to troll), most enemy teams won't know how to or be too lazy to work together to prevent a Teemo from taking over a map entirely.

Pros
1) Monster at vision control
2) Strong poke lane as Teemo's blind q allows him and his adc to harass
3) Requires very little gold to stay relevant utility wise, hence even if he and his adc lose lane he will always be able to offer core things desired of supports.
4) Does tons of damage to the enemy team relative to other supports, and if he gets ahead his shrooms become shrooms of doom (often will do more than a lot of the carries).
5) His shrooms can result in a lot of easy kills because of the enemy walking through them at a bad time.

Cons
1) While his shrooms result in aoe slows, and his q is a blind, his ability to provide peel and hard engage is generally conditional on the enemy walking into his shrooms (however, remember some areas are hot spots for traversal).
2) He is not tanky hence suboptimal in a team without tanks (no frontline to defend squishies, particualrly the adc)
3) He can be considered a poke support hence he will struggle against sustain supports who can just counterpoke and heal up any harsas you do, resulting in you and your adc ending up with less hp.

- Map of hotspots for traversal, where to place shrooms essentially (early game, mid game, late game)
- Detailed guide on how to play during the laning phase, also the level 1 invisible cheese (i.e. stay stealhed in the middle of the bottom lane, and get a good combo on the enemy adc when they enter and start to farm at level 1 - with good team work him + his adc can do a big chunk of damage and maybe even result in summoner spells blown)
- Item discussion (in particular the mana items. Tear for cheap can sustain you for ages and allow you to spam his w allowing for quick movment and also allowing you to focus on getting the other core items such as sightstone and liandries.
- Ability discussion - Q for blind obvious, however w vs e.. e for more damage in lane, w for easier warding and shroom placement.

Vision Matters

League of legends big shots always talk about the "vision game", and how if you are able to win the "vision game", then often that means you will win the "game" ifself.

In simple terms, knowledge about the movements and locations of your enemies can allow your team to make plays with less risk, taking objectives that they know the enemy can't contest without risking a lot, while also limiting downside risks, such as your team getting into bad fights outnumbered, or your team moving to respond to the attempt at an objective by an enemy.

You often win or lose games because of a lack of vision, and certain team comps, pick comps, require good vision control from the get go.

Imagine playing a game for 50 minutes long, both teams have even gold, then as the death timers have gotton super long, your adc gets pulled into an unwarded bush by a blitzcrank. Or imagine you and your team mate chasing after an enemy squishy only to suddenly find yourself 2 vs 5 as the enemy team has collapsed on you, whereby they then take baron as a result of removing you 2 from the map for a period of time.

Or you are a Rengar and you see that the enemy vayne is bot lane farming and pushing a little too deep, and vision in the jungle near bot side shows that there's nobody else there - you ulti and kill her and then force a fight after 4 vs 5 which you are likely to win as they don't have an adc.

Games can be won or lost solely as a result of a lapses in vision.

Choosing the right support matters in winning the vision game

As a support, you are often left to win the vision game entirely by yourself because the rest of your team are ******s. Expect your team mates to not buy wards or upgrade their trinkets by default.

However, the same will be true of your enemy team (they won't buy wards either), and hence one way for a support to climb is by essentially winning the vision game against your opponent support.

Picking the right supports to do that however matters.

Champions like Janna have dominated consistently for years (highest win rate support for ages across all elos), often this is put across as a result of her kit being OP for disengaging and etc etc, but one thing rarely mentioned is her ability to win the vision game.

You'll find her #1 in stats and rankings like "wards placed", "support that deleted the most wards", but also #1 in "least deaths".

With both movement boosts and disengagement tools in he kit, she can ward easily, fast and do so without dying - she can with little risk enter deep into the enemy side of the map and jungle and ward, giving her team useful information about enemy map movements, particularly in the early game jungler movements.

Often one of the best ways to shut down particular enemy janglers who can't necessarily be stopped by simpler warding such as stealth junglers is to ward deep and locate them when they are unstealthed so you always have a general idea of where they are e.g. Rengar, Evelynn.

A Soraka cannot always get deep vision as easily as she's squishy with no dashes or movement boosts and if caught will struggle to escape, a Leona that is tanky might manage to survive a little longer but given her slow movement will still probably just end up feeding as the enemy mid laner roams down to help the Rengar finish you off.

Some supports essentially are better at the vision game than others.

Luckily for you Teemo is an OP piece of **** at the vision game.

10 minute uptime shrooms

Teemo's shrooms last 10 minutes from the get go and give a reasonable chunk of vision of their surrounding area.

With only a few shrooms Teemo can secure vision of a very large area for extended periods of time for $0, and given his R's cooldown is relatively low especially if you build cdr, he can easily replace them when they pop or push deeper into enemy territory adding layers of vision overtime.

Teemo can always keep vision around important objectives secure, and given magic pen builds can often win team fights before they begin because the enemy squishies walk through his shrooms to contest. Or his shrooms can even be used as a tool to engage, given that they slow allowing your team to land abilities and skillshots easier.

Given their nature and human nature in general, after suffering to them enough times the enemy team will likely avoid entering those parts in general; this will be especially true in soloQ, and even moreso in low elo soloQ because of the team work and costs that will be required to handle them.

Because he isn't such a common pick, and more importantly the few Teemo supports there are don't take advantage of this feature of his kit (usually they troll, don't bother buying sightstone or care about vision at all, build bizzare builds etc), most players you meet won't know how to handle them or at least be too lazy to develop and implement strategies to prevent a Teemo from taking over the entire map.