"it's really just an internet thing" is just completely wrong.

If I'm a top and am not comfortable bottoming, then no matter how attractive the guy I'm not going to want to get with another top (yes, I'm aware of frotting and giving head, but sometimes you want penetrative sex yk). So I put that on my profile, to help ensure I match with someone I'm going to be sexually compatible with.

Beyond preventing wasting time, it also helps give clear boundaries and ensure that hookups are safer for both involved.