One of my best friends is a corporate manager for one of the largest chains in the country that has plans to implement a similar strategy. The reason is like anything else--to cut costs and maximize profits. If you have a cart that can scan as you go and keep a total, all you need is a credit or debit card tied to your store savings card and you can go right from the shelf into a bag. When you're done, you walk out the door.
This does have the negative side effect of making all of those checkers and baggers obsolete (and therefore unemployed), but from a shoppers perspective it's great. You can just walk in, get what you need and leave. No lines, no waiting, no stoned bagger putting the liquid laundry detergent on top of your carton of eggs.
One drawback you might notice is that it depends on shopper honesty to scan everything that goes into their cart. In test stores, apparently, it works quite well as most shoppers are either too honest or too paranoid to try to cheat the system. There is a degree of loss, but it's more than compensated for by the savings on payroll that used to go to pay the extra checkers and baggers. Additionally, they have relatively short term plans to implement RFID (3-5 years out) which will make the system much effective. The stores aren't stupid though, they plan on rolling these out only in stores in nice neighborhoods. There, the rate of theft is projected to be low enough as to not impact the bottom line.
If you already use a shopper's card or pay with a credit card, the supermarkets already have all the data on your shopping habits they need. If you're too paranoid to use either of those when shopping, then this new system won't impact you at all. There will undoubtedly still be a line for you to stand in to pay cash.
This is a good time to punt work.