c/r'er thinks you have jackshit but has something himself.
Normally most of these players suck too much postflop to really raise your cont bet anyway, so when you call the miniraise (as you prob should anyway) they get scared on the turn when you get aggro at them again.
Of course its also a way fo playing the nuts when your opp is way behind... but thats a pretty obvious line from a donish player
being cold called or miniraised by a good/thinking player is much more scary here imo.