new player with question about roles

hi guys i am rather new to this game, only been playing LOL since late Nov last year use to play a bit of 3 Corridor years ago when Warcraft3 was popular (if anyone remember those times did play about a week of dota on War3 as well, then gave up LOL on the other hand was more interesting for me for some reason anyway, i have a question that's been in my mind ever since i started playing LOL **why is that, each line are only performed with a particular type of champ?** tops are usually tanks or melee, mids are high dmg AD melee/AP mages, and bot are ranged ADC and support why is that? for what reasons are those roles dedicated to only those lines?? for example, before shotgun bob {{champion:104}} got nerf, he was a good all-rounder IMO apart from jungle, which i still don't understand, i could take bob in any line and have a good game but when i call top with him, lots of people be like "WTF you doin on top line with a ADC" or "don't play top with ADC, we need tanky champ on top" same thing with Ashe, i play her alot, but if i call top with her, even in Co-op AI team mates starts asking questions O_O but, if enemy team have melee on top line, or any line for that matter doesn't that give me some advantage using ranged champ against melee? might be a stupid question but i really like to know the reason behind this "rule" any answer would be appreciated, thanks :D
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