Let me attempt to illustrate an answer to that:
Let's say that you live with your wife, and you are just a generally awful person. You drink heavily, have no patience, and were too cheap to buy a punching bag, so you just use your wife.
One day, she does something to really piss you off, and so you, being a generally awful person, decide to kill her. So, you run out to the store and buy a gun. With a waiting period, you'll have to wait, come back and pick up your new gun, then go home and do it.
The idea is that that moment of blinding rage will pass, and you won't kill someone. Unfortunately, gun control advocates fail to realize there's a million ways to kill someone. Will it stop someone from going out, buying a gun legally, then going home and committing murder? Sure it will; but it won't stop the murder--it'll only stop the legal sale of a firearm to someone who has malicious intentions at that very moment.