Coming from a support main going into my third season and has peaked at Plat V, Support is the one role where you HAVE to rely on others. You aren't the damage dealer, you aren't the massive tank, and you aren't the person people usually fear. Your entire job is to make it so the enemy team has someone to fear, but that person is also your biggest kryptonite.
You can feed your laner as much as you can making them go 9-0-2 in lane or even something more ridiculous. Yet this doesn't make them a better player. It won't make them position any better and it won't make them understand how to rotate any more. They are just as good as they were, but with more damage and making it more important to protect them now.
Personally I rarely lose lane, but anyone who knows anything about this game then you know lane doesn't mean anything if you can't keep up later. If you can't position well, teamfight, rotate, or close out the game it doesn't matter how well you do in the first 10-15 minutes of the game. You are still bad at what you do and it won't change just because you have some gold and a lot of kills. Every other role has something about it that can carry a team. You either have the damage carries in bot and mid lane, tank/initiator top and jungle, and the junglers who can appear from no where and deal damage while feeding their laner. Most of these champs have damage or superior tankiness that can change the fight or put matters in your hand.
Now thats the downside, but in the hands of a good team a good support is a literal god. If your team understands the basics and only makes minor mistakes a support will make or break the game. Where they ward, when and where they choose to use their utility, and how well they peel can change everything. It is a massive double edged sword that is based around not only your own skill, but the skill of your teammates.