This is very much "it depends" on the opponent, board texture, etc. If you're dealing with someone who plays too many pots passively, then it shouldn't take more than an orbit or two to get a good feel for what's he's up to.

USUALLY it's an attempt at pot control with a weak hand or draw.

SOMETIMES it's a value bet with a very strong hand to attempt to at worst get a call, at best induce a raise.

Given how that the turn completed the flopped flush draw, there is often merit to calling to see a river card + action. However, with the nut flush re-draw, our hand is probably too good to not put in a small raise for value (a little less than a buck or so.)

As played on the river we figure to be good too often and his play is pretty sketchy so it's a pretty clear raise for value even though you'll feel stupid and see the flush sometimes. You'll also feel smart and get called by T9 sometimes...