Tahm Kench is a tanky bully in lane that can play aggressive or defensive depending on the lane. He pokes down enemies, and can save his adc from a hairy situation. His great sustain keeps him ready to fight at any time. He provides great global coverage with his ult and can gank well. Tahm Kench will rarely not be useful because of his tankiness and he is certainly a great support in my mind.

Tongue Lash
-Great poke early and mid game
-Adds a passive stack
-Good slow
-Stuns if target has three passive stacks

Devour
-Great for chases because of movement buff when ally in mouth
-Can save teammates from hairy situations
-Tahm can eat and throw people over certain walls
-Can lock down an enemy carry

Think Skin
-Adds a second health bar to Tahm
-Provides good sustain in lane with grey health turning into a small heal
-Can dive towers very easily with this
-Helps to survive long chases

Abyssal Voyage
-Provides Global presence on the map
-Helps team get to lane, dragon, roam, or to save a tower
-Can be used to heard enemies
-Converts health into damage (Passive)

Your early game focus as Tahm Kench is to peel for your adc and to keep him alive even if it means risking your own life.
PLAYING AGGRESSIVE
Tongue Lash (His Q) provides great poke and also provides a passive stack for engages. With more aggressive adcs, your slow is helpful and you can also take an enemy out of the battle for a few seconds with his Devour (His W) aswell as doing alot of damage. With these early engages, your Think Skin (E) is helpful for extra tank and provides a greater survivability.
PLAYING DEFENSIVE
Poking is still necessary, and is helpful for when ganks come; however, being harassed heavily it makes it hard to poke. When playing defensive it is important to keep your adc alive at all costs. Use everything in order to keep your adc doing well because even if you behind you are great of global mobility and tanking.

Helping your team is key. People will be thankful when they get a one way ticket out of danger with Tahm's Devour. Ulting teammates to save towers, to dragon, or to get back the lane is great to keep your team from falling behind. Tahm Kench provides global pressure and creates space when split pushing with an adc or catching an enemy out. The main goal during this period is to ward for your team and finish core items.

Late you job is pretty much the same; however, you are more tanky and your Think Skin provides an extra health bar. Furthermore, you do a pretty good amount out damage since your ult passive makes you do more damage with the more heath you do. With your devour and tongue lash damage aswell you are a major theat to squishy enemies.

As Tahm Kench your main purpose is to tank up hits and focus the threat and to take them out of the fight by Devouring them. If you are being focused then it is good because you can easily tank and if someone else if being focused, eat them and you will probably be focused. Eating a carry that is low is important because it can give them the time to re situate them self and can clean up when everyone is low. It is important to know how to manage your devour and to know when it is best to devour and enemy or ally. The more you practice the more you will know.

Items are very self explanatory and focus on making Tahm and unkillable tank for your team. Analyzing the threats are key. If there is a fed AP Carry build magic resist and if there is a fed AD Carry build armor. Furthermore, Righteous Glory is help for chases, engages, and escapes; it also provides health and mana. Health is always good especially on Tahm because of his ult passive that is why warmogs is not that bad.

The runes focus more on the tanky aspects of Tahm and improve his early as well as his late game with heath and armor. Moreover, the masteries focus on tankiness and keeping your adc alive. The cooldown reduction helps with poke and such. Bond of stone is important for reducing damage afflicted on your allies. Other things like bandit help you generate gold, tough skin for sustain and tank, and insight for reduces cooldowns on summoner spells.

Tahm Kench is a great support that is tanky, but can also deal some damage. He works well with many carrys and is helpful throughout the game. You are definitely noticed if you play smart and correctly.