In Poker, What is the Significance of "Betting on the Come"
It can be a bluff or it can be a calculation.  If you have QJ and the board shows 10 9 3 then you have 2 overcards to the board and an open ended straight draw.  If you have 4 cards to a flush you are probably favored nearly 2 to 1 to an over pair even.  

If you sense weakness in your opponent you might be able to scare them off of a hand like two 5's, that while leading as it stands, could be behind if you even have a 10 or 9.  

Your opponent could also have a single ace or of the suit that has 2 on the board you do not want that over card pair or another card towards his flush to come.  So you can bet to scare him off now.  

Generally betting the come you really want to take the hand down right there.  But you do not want to commit yourself to playing a hand out if the next card doesnt come in your favor.