this is my personal experience, not stating I'm right or something. But you should really only play support if you feel like that would benefit your team. There are 2 ways of benefitting from that role: * you're a really good support * you aren't competent enough to carry games

so basically if you're a decent player for your elo and know how to win games on carry roles, you'd probably want to play a carry role and let the weaker players to get a support role. Because you know, if you occupy a support role, some player who is only good at supports (and picks Support/Fill while queueing up) may be autofilled to mid, ADC or jungle and fail miserably.

but if you're a weaker player yourself, playing a support would mathematically increase your chances to get carried. In this case if you pick a support role, some decent player would have much less chances to be autofilled as support and waste his carrying potential.