I've done it out of curiosity.
I play microstakes and sometimes run into people who will donk lead for 1 or 2 bb post flop on every street in raised pots, even in 3 bet pots and multiway. I've been curious and called with nothing before.
One answer is to raise as it looks weak, some players like to do it as a bit of a feeble bluff attempt.
Some players do it to induce a bluff, and they were actually strong, either calling then betting big on the next street, or re-raising.
Other times it's a player on a draw, thinking he can put a min bet out as an attempted block bet on the flop/turn to see a cheap card.
It can be a player with a mid-strength hand who just doesn't know where he is at.
Some players just don't seem to understand how to size bets at all.
In that sort of situation I've called before just to find out which of those, or anything else they might have been doing so that next time they do it I might better understand their approach.