Flash sales are becoming more popular with both retailers and consumers it seems.Recent reports have shown that by offering time limited deep discounts retailers can build both brand loyalty and also unload surplus stock:

Over three in five Americans prefer to spend their hard-earned dollars on discounted items online than in retail stores, found Kelton Research's study. Hardly surprising when you consider consumers saved a combined total of around $500 million by shopping via online sales in 2009. That figure is expected to double this year.
One of the most popular ways in which consumers get the best deals is via "flash sales". These limited-time deals offer attractive discounts averaging 50% off retail prices.

The defining feature of these "flash sales" is that they only exist for a short time. While consumers might need to get an online coupon or, in more extreme circumstances, receive and invitation email, they can get those great discounts.

From hte retailers point of view the short time scale that the offer is made for means that they're not liable to cannibalise full price sales by offering the discounts. And that is of course what retailing is all about. Making as many full price sales as you can and only offering discounts to those who would not pay full price anyway.