Mainly you want to avoid your credit card issuer freaking out when they see a bunch of small charge. If they reverse a purchase you might be locked out of your account for the rest of the sale or even longer. But it also useful as a way to budget and keep a cooler head. You decide to spend X so you put X in your steam wallet and when X is spent that it.
from reddit: the front page of the internet http://ift.tt/1vyC7OW