Online retailers have massive sales. It's a follow-up to our "Black Friday". There are certain deals that you (supposedly) can not get otherwise. Usually, there are discounts on shipping and bonuses (like, spend $25, get a free 'something'), as well as the sales.
In my opinion, it's pretty much a ploy to boost consumerism, but is supposed to be a way to save money on gifts & kick-start the Christmas season.