Sometimes you can save money....
you have A9s on a T92 flop, opp has Cowboys (you called from the BB cos KK raised from the button

)
u chk, he might chk behind you trying to trap (ok so he might just flat trying to trap to)
turn is a 7
u chk (pot size control, dont wanna play a big one with 2nd pair), he double checks really setting a trap
river is a 5
as he's chked along with you you might feel your 9s A kicker are good, and bet 2/3-3/4 pot trying to pick up the pot without a fight (unless u think he might call if he has a 7, thinking you hit the river, then its for value) --> TRAP SPRUNG raise on the river.
If he does call your flop bet, u can just chk it down and fold to a bet.
Had u bet 1/4-1/3 pot on the flop, more than likely he's gonna raise especially if there are big draws out...
if they are on a draw, do you really wanna give them a free turn or river?
In the hand I described that originally got all this going, I only bet the river in that hand, cos as I said, I knew the player pretty well and was pretty sure he'd call with A high on a 9 high no flush board...
Im not saying you wanna use these kinds of bets all the time, but they definitley have there place in your poker bag of tricks...