by putting the limit order in which usually fills you get to save some money. but may lose out if it goes up from where you started it. but i like them because i dont want to sit in front of the screen and mwait for the best price. alot of times i am willing to buy at x but not at x+1/2, so i put in a limit order and walk away and do something i like. plus it makes you feel like you got a bargain if you get in a little lower than the ask.