Most teams follow this team composition. What are the reasons for it? For example, why don't you typically see AP top with bruisers in the middle or a ranged AD in top-lane?
Mid champs exist for a couple of reasons. Mid champs are normally blue dependent and have a very strong early-mid game. Mid therefore is good place for them since:
- It's always closest to blue
- They can roam to top and bot for ganks plus help the jungler if necessary since they are central
- It's closer to dragon
The reason for having two bot goes way back to the fact that dragon is the home of bot lane (therefore you wan't the two champion lane closest to it). You don't necessarily have to have AD/support there however it has been proven game after game that baby sitting your AD with a support bot lane works really well. Since AD carries don't scale as well with level as bruisers/caster it also make sense to just give them the farm but not the full XP.
As for top champions they are normally beefy or have escapes since they tend to push up very far and can be very exposed to ganks.
As for which champions go into each lane that is very dependant of the current meta and will likely change in the future.
I won't go into explaining why you have a jungler but it's been proven that having one is a lot better than not.
AP does massive burst damage and that will allow them to gank bot or top easily and burst down the enemy champions.
There is usually someone in the jungle because it allows one person at top to level up more quickly than if there were 2 people in that lane and the jungler usually has cc which allows to to gank any of the lanes.
Support is usually sided with ad range on bot because ad range is weak early game and can do lots of damage but is hurt and killed easily. That is why support is brought int to give them shields/heals/buffs ect... This allows the ad carries to continue to farm up to try to become powerful enough to win the team fights.
The player on top gets XP faster because they're alone and they get the last hits so they get more money. The Jungler gets the jungle buffs and ganks the lanes. The Mid gets XP and Last hits like top. The bottom Carry gets XP with the support so they level slower, but the Support lets the carry get the last hits and usually kills for money and they keep them in lane.