To be fair, and this is rare, I main mid and support. As a mid player, I have 2/3 that I am confident with, and others that I am only Meh with. Sometimes the enemy picks a champ that counters pretty much all my picks (or the jungler is such a hard counter that I am better off picking something else anyway, like Galio into Udyr/Braum support). So I pick someone who I am REALLY good with, and focus on roaming or stalling until mid game, rather than someone I can't use very well, lose anyway, and end up not being able to make a comeback.
Anivia into Syndra for example is suicide, so I'd rather roam with Taliyah than try to fight her with something I am less confident with, whereas champs like Kassadin outroam me and outtrade me, so I just go with picking Anivia and just playing under tower is probably best bet, rather than going something like Galio who might be able to win trades, but will get ganked easily, run out of mana often, and I won't be expecting ganks against a Kass unless I have Vi or something as jungler.
These are mostly examples using champs I play, I don't play ad midlaners and a lot of other champs that would be safer
edit: you did say "if you don't main" to which I should just say those people are "Yolo" lol