At TRU we are supposed to ask pretty much every customer if they need help. Some people are hard to read, they may look like they are doing fine, when they are really having trouble and don't want to ask for help. You wouldn't believe just how many people today that needed help but didn't approach me, although I was only about ten feet or so away. I approached them and sure enough, they needed help.
That, and it's also to deter thieves, as already stated.
There's no reason to be a dick, as the people are just doing their jobs. As far as being asked by every employee, that is a bit excessive. We
try to assign certain parts of the store to each employee to keep this from happening, but sometimes people get asked several times.
EDIT: I hate when I'm checking out and the cashier ask "did you find everything you were looking for?" Uhhh....if I didn't do you think I would be leaving?
Once again, this applied today. I was working the pick-up desk (we're still on the old ticket system) and I always ask the customers if they found everything they needed, even though they've already paid. Most of the time the people respond with a "yes" but every so often like today, there was a lady who couldn't find a certain game (can't think of the title), as there were no tickets for it. I told to let me look in back and sure enough, we had several copies of that game. I took it over to service for her and she bought that in addition to her prior purchase. That customer let even happier, and would not have been fully satisifed had I not asked her.
Remember, what may be an incovenience to you may be of great convenience to someone else.