Legit question. At least in low elo. A role that is dedicated mostly to support one single damage dealer (maybe apc too, but mostly adc). I mean, you can easily pick a poke ad\apc and if you survive mid game instead of 200-300 shield and maybe an engage you have another big dps threat. I guess in higher elo it might not be the case since support is the one who isn't getting much gold and people make less mistakes there, but I'm speaking of gold and lower.
Lugg2 votesThe ADC is probably the most important role late game. They do amazing sustained damage and take out the enemy team. Because of this, they are usually focused more and that is why they need a support to protect them while the scale up. This also speeds up the scaling since they don't have to spend gold on wards, potions, etc. and can just get their damage items. This is why they get the support.
Also, supports bring abilities that help win teamfights and they can roam and help gank other lanes just like the jungler. Every great team has a good support.
Maximum Zilean1 votesBecause in most cases, keeping the AD carry alive is better than having more dps.
There are some that do both nicely though and are viable supports. 
And most supports bring stuff other then shields / heals. Usually amazing CC and fight control.
Niaphim1 votesSure, but this is my question. Instead of supporting one adc you have 2 and can even sacrifice one - you'll still have damage from the second
Sir Yamazuki1 votesSure, but this is my question. Instead of supporting one adc you have 2 and can even sacrifice one - you'll still have damage from the second
Unless you're playing cass support, you aren't dealing constant damage, which is why adcs get a support, and usually the mages that end up as a strong support have cc/utility along with their damage.
Lugg3 votesYou end up with 2 people trying to farm a single lane. That leads to neither of them getting enough farm, which leads to being behind in experience and items. This actually cripples 2 people (40%) on your team and that is very hard to overcome.
Catastrop1 votesBecause when your ADC gets ganked over and over because there was no one to buy a Sightstone or upgrade their bloody trinket, they're going to bitch and moan to their jungler that he/she is doing a bad job. Plus you generally want someone who can offer utility to the team and not pure damage. Why? Because if they had a support like say 
who can wreck havoc with their CC and you have two ADC's who bring little to no CC, you're going to find that often times they can't utilize that damage due being on lockdown.
Reaper Soraka1 votesIn the lowest of elo, you probably don't need a support so much as someone who can ward, and react to enemy movements around those wards. Teemo, Panth, even a cait support is fine if it can do it's job.
Aeolian Melodies1 votesBecause one farmed carry with a champion whose kit is dedicated to helping him is better than two half-farmed carries.
Koala Frenzy1 votesLegit question. At least in low elo. A role that is dedicated mostly to support one single damage dealer (maybe apc too, but mostly adc). I mean, you can easily pick a poke ad\apc and if you survive mid game instead of 200-300 shield and maybe an engage you have another big dps threat. I guess in higher elo it might not be the case since support is the one who isn't getting much gold and people make less mistakes there, but I'm speaking of gold and lower.
Damage isn't the point. Supports are there to protect their adc or team throughout the game. A farmed protected ADC outputs much more damage than two moderately farmed carries that die instantly.
Binxy Prime1 votesMost of it just has to do with how to efficiently share the gold on the map. Supports do a lot with very little gold, their CC is really powerful and some get tanky really easily cheap, like Leona. This allows you to function on a small amount of gold. In some cases I would rather have the CC of a leona for example than even another full build AP carry because you really need that cc to stop people from training your team.
Abyssphere1 votesThe support role exists to distribute gold in the most efficient way - mid and top farm solo, jungler takes camps and ADC farms solo while the support (the person who by virtue requires the least gold to effective) relies on their gold generation item, masteries, runes and so on for their own income. Double ADC or just two carries bot in general fundamentally doesn't work because you're no longer distributing your gold in the most efficient way - if one of your ADCs isn't taking farm, they can't use a gold gen item because none of them synergise with an ADC/you don't have room in your build for one. So any other support would be making more money than you, even though they are able to function with a lot less gold than you need to work. If both ADCs are taking farm, you're not distributing your gold effectively either because neither of your carries the amount of gold they need - B.F. Swords are massively delayed, for example. The end result gold-wise is either one of your ADCs ending up worthless because they don't have enough gold to do any damage at all (in which case any support at all would be far superior) or both or your ADCs having a moderate amount of gold but both of them being incapable of 1v1ing the enemy ADC or coming close to carrying.
Aside from not having gold problems, supports contribute a huge amount to damage (and/or damage mitigation) in team fights. Like, you massively underestimate what your support contributes if you think "a shield and maybe an engage" is all they do. I mean look at Lulu. She shields you for 300, she polymorphs the Rengar that was gonna do 2k damage to you, she slows him to ensure his death (meaning all the damage that was done to Rengar was essentially down to her, that's about 2k damage that probably wouldn't have happened if you had another ADC instead), she ults giving someone about 700 HP and knocking up anyone else who's around the person she ults (giving time for your team to get free damage on them) and finally she slows with her Q, uses any of the variety of actives/passives a support has access to, and so on. The support in almost any team fight will mitigate/cause more damage than a second ADC could dream of doing, even if they somehow survived the entire fight without a support to peel for them. And they do that without causing problems to your primary carry's gold income.
I can't quite put into words just how much you screw over your team by replacing a support with a DPS source. I mean if anything this applies MORE to low elo than anything because at that level your ADC isn't CSing as well as they would higher up, so you have even less potential income to be wasting. And this is before we get into the fact that low elo players will have much worse positioning so they need support peel more than anyone. But you seriously need to look at what everyone brings to your team beyond damage numbers, and I don't just mean supports here. Otherwise you'll never understood the value that utility-based champions bring to your team in any role.