This might just be one of the best League Meta question ever asked
(certainly my favorite one).
- Sustained Damage
- He functions as a backup plan and snowball at the same time.
I'll try to answer your question with some additional information to the competitive metagame.
About the Meta
Now the meta game is usually controlled by buffs and nerfs on riots side. In very rare occasions there are some pro players that influence the meta with weird picks by discovering a strategy on a champion that was overlooked by Riot. The biggest factor to changes in the meta game are however not champion but Item changes. Items like Cinterhulk, Feral Flare and Force of Nature have shown that the changing, introduction or removal of Items can change the FOTM (Flavor of the month) Champions completely.
But the thing is: while all these things have a direct influence the meta, the changes aren't that extreme. Since the end of Season one the meta has become fairly stable. In the pasts 5 years toplane has always been the farm lane for Tanks or Bruisers that scale well into the lategame. Midlane was usually dominated by Mages or Assassins that have a huge impact on the midgame. Now the viability of a certain jungle and support pick is usually determined by the current meta in the toplane and midlane. If there is an assassin meta you will usually find junglers and supporters that peel for the ADC while during a poke (mage) meta you will find a lot of engage supporters and junglers. Of course they're also influenced by item/champion changes like every other lane but those two roles are the ones that aren't easily changed by simple item changes and/or buffs.
As a sidenote: Yes there have been some weird picks in the past like Quinn Top or Soraka mid but those aren't really meta, they're just situational counters or surprise picks.
But as you said in your Question: The ADC always stays.
Why is there always an AD Carry?
To understand why the AD Carry is present in every meta you have to understand what an AD Carry actually does.
It is the Attack Damage Carry which means that he will always bring physical damage to a teamfight (sometimes hybrid in the cases of Corki/Kog'Maw). Now the carry in Mobas usually stands for a character that can dish out a lot of damage but is fairly squishy (easily killed).
But why don't we just send an AP Carry bot with a supporter?
There are two reasons for this:
- There would probably be only one or two physical damage dealers in the team.
- There aren't a lot of AP Carrys that have sustained damage, and those are better played mid.
And the 2nd point is the most important one. Sustained Damage. The ADC is the champion that can dish out high damage in any situation for a long period of time if he's well protected and positioned.
Tanks usually have low sustained damage, Mages have either burst or Poke with high cooldown, Melee ADCs are very quickly countered by CC and Supporters are usually just here for initiation and protection. The ADC is the backup. He can kill the frontline with kiting where a Mage would just waste cooldowns or where a melee ADC would just get CCed and bursted. He can work his way through the teamfight with constant high DPS as long as he gets protected by his own frontline. He's also great at cleaning up the remaining teamfight and at getting objectives. If the ADC is the last man standing after a teamfight he can usually kill 1-3 enemies who survived the skirmish with half HP.
Now obviously there are different types of ADCs and this is where the actual meta changes show themselves. In Assassin metas you will usually find more bursty and mobile ADCs whereas in tank metas the high-range ADs are dominant. Bursty ADCs like Graves or Corki are almost hybrids of other roles but the fact that they build Attack Damage and Crit guarantees that they will have some sort of safe sustained damage.
Why do normal people play with an ADC?
There are two main reasons to this:
- The community is extremely Influenced by the Professional Scene.
- The reasons above also apply to the normal community.
I'm pretty sure that if there are multiple matches in the LCS where no ADC is picked, the majority of the Community would stop playing ADCs, regardless of whether it's good or bad. But in general: If you're ahead with an ADC and you know how to position yourself it might just be the easiest role to carry with. Having a fed ADC in your team increases your chance of winning drastically.