If you're a bad support you do nothing. If you're a good support you:
Help your adc cs under turret, increasing their farm and improving their dmg.
Know when to exhaust high priority targets.
Properly use your extremely powerful actives.
Get vision of the map for your team.
Roam to get kills in other lanes like a jungler would.
If you're really good at support, you're usually helping the team shotcall because supports should have the highest map awareness.
Supports tend to have very good kits. Even their basic abilities can change an entire teamfight.
And they manage to do all of this while people consider them their bitch, and they get very little gold. A really good support can easily set up kills or wombo combos that win the entire teamfight.
Either with a form of hook and extreme cc like a tank-
Blitz, thresh, nautilus, leona.
Or large impactful abilities/ ultimates.
Nami, Janna, Rakan, Maokai, etc
My fiance played this game for four years. I kept telling him to support. He refused. He could never pick a main or a role and bounced around aimlessly for FOUR years.
About two months ago he finally caved. He now loves it and is maining Rakan, and Velkoz.
Successfully get a carry fed when they can't even last hit their cs and then say support is useless.