Mages supports like Annie, Brand Lux and Zyra will have a easier timer carrying (if you're actually good) since they are less dependant on their adc/team.
Good Thresh but bad teammates? Land a hook and perfect initiation, only for your team to leave you to die since you can't kill the enemy by yourself.
Good Annie with bad teammates? Who cares, one-shot their squishies. Which also might help them catch up since you help them get a lead, even if they're bad.
There's nothing worse than playing an initiating support, only to have your adc afk farming while you engaged on them.