Generally you'll see something like this.
Top - Something that's fairly tanky and/or has escape abilities, often with sustain. This is because top is the easiest lane to gank and isolated from the rest of the map, so picking a champion with a high chance of surviving or even thriving alone is a good idea.
Examples:
Vladimir,
Kennen,
Udyr,
Riven,
Shen,
Jax,
Olaf
Mid - AP, because they need to level up quickly in order to become strong. Mid is the shortest lane and thus usually the hardest to gank, which is good for AP because they're mostly very squishy without innate escapes. Plus being in the middle of the map allows AP champions the chance to roam to other lanes and pick up kills, which is something they're often good at.
Examples:
Karthus,
Ryze,
Cassiopeia,
Ahri,
Morgana,
Vladimir,
Veigar
Bot - AD and support, because AD is very vulnerable to being killed in any other lane. They need the help and the warding from supports to survive early game and they don't really depend on levels, mostly money. The reason they usually go bot instead of top is for greater numbers around dragon. Supports build gold items and don't take cs so that AD can get maximum farm.
Examples:
Graves,
Corki,
Caitlyn,
Sivir,
Kog'Maw +
Janna,
Soraka,
Taric,
Leona
Jungle - Usually someone tanky, though that's mostly because all the good junglers are tanks so you don't really have much choice. You generally want them to have some good CC for ganks and good AoE damage helps for clearing the jungle.
Examples:
Nautilus,
Nocturne,
Dr. Mundo,
Shyvana,
Udyr
Top - Something that's fairly tanky and/or has escape abilities, often with sustain. This is because top is the easiest lane to gank and isolated from the rest of the map, so picking a champion with a high chance of surviving or even thriving alone is a good idea.
Examples:







Mid - AP, because they need to level up quickly in order to become strong. Mid is the shortest lane and thus usually the hardest to gank, which is good for AP because they're mostly very squishy without innate escapes. Plus being in the middle of the map allows AP champions the chance to roam to other lanes and pick up kills, which is something they're often good at.
Examples:







Bot - AD and support, because AD is very vulnerable to being killed in any other lane. They need the help and the warding from supports to survive early game and they don't really depend on levels, mostly money. The reason they usually go bot instead of top is for greater numbers around dragon. Supports build gold items and don't take cs so that AD can get maximum farm.
Examples:









Jungle - Usually someone tanky, though that's mostly because all the good junglers are tanks so you don't really have much choice. You generally want them to have some good CC for ganks and good AoE damage helps for clearing the jungle.
Examples:





Ranged does not necessarily go mid, there are melee mages as well.
The way the current metagame is centered, it looks something like this:
AP caster mid: They scale better with levels, as their spell combos do more damage the higher in rank they are.
AD ranged carry bot: They scale better with creep score and gold. Most of their damage is from auto attacks, and buying items increases their damage.
Support bot: To protect the AD ranged carry. They scale pretty well with items, however, they don't need items to achieve a lot of their effectiveness. Bot lane is also the closest to dragon, which is an important objective. Thus, having someone to buy wards to make sure the enemy isn't doing it makes bot lane an ideal lane to have a support in.
Bruiser/Tank top: Teams generally function well with at least 1 tank and 1 tanky but not complete tank character. Top lane is the longest lane, and the least likely to be warded, so more resilient characters tend to play in this lane. If someone like
Malphite were to get ganked, he could probably escape because of his innate tankiness and CC. Someone like
Ashe on the other hand is too squishy and would probably die too quickly.
Bruiser/Tank jungle: Because of the damage one takes from the jungle creeps, and the fact that you get the least amount of gold from the jungle creeps, tankier characters tend to take this role. People who are effective with few items, yet can still clear the jungle and gank while not having to recall often are better in the jungle. Tankier characters tend to have more CC for ganking as well.
I could probably think of more reasons later, but that's what I have for now.
The way the current metagame is centered, it looks something like this:
AP caster mid: They scale better with levels, as their spell combos do more damage the higher in rank they are.
AD ranged carry bot: They scale better with creep score and gold. Most of their damage is from auto attacks, and buying items increases their damage.
Support bot: To protect the AD ranged carry. They scale pretty well with items, however, they don't need items to achieve a lot of their effectiveness. Bot lane is also the closest to dragon, which is an important objective. Thus, having someone to buy wards to make sure the enemy isn't doing it makes bot lane an ideal lane to have a support in.
Bruiser/Tank top: Teams generally function well with at least 1 tank and 1 tanky but not complete tank character. Top lane is the longest lane, and the least likely to be warded, so more resilient characters tend to play in this lane. If someone like


Bruiser/Tank jungle: Because of the damage one takes from the jungle creeps, and the fact that you get the least amount of gold from the jungle creeps, tankier characters tend to take this role. People who are effective with few items, yet can still clear the jungle and gank while not having to recall often are better in the jungle. Tankier characters tend to have more CC for ganking as well.
I could probably think of more reasons later, but that's what I have for now.

If I helped you, click that +rep!
top - bruiser
jungle - tank/support
mid - mage
bot - ad carry/support
jungle - tank/support
mid - mage
bot - ad carry/support
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