Those Annoying Alarmy Things at the Front of Most Stores

Does anyone know why they have those magnetic alarms at the front of department stores?

Does anyone really pay attention to them when they go off? When I've been in stores such as Best Buy and Target, I have witnessed, when people walk out the door through the security gate and set off those alarms...nobody even looks. Some of the stores that have those theft indicators even have security guards, whose sole job it is to stop you from stealing and they don't even look up when the annoying beeps sound. Maybe those guards know that the person at the register usually forgets to demagnetize the security tab or that the demagnetizing machine at the register usually doesn't work. What's stopping people from taking things (aside from general honesty of course)?

I went to a club once that had the same kind of thing only it was a metal detector. It was 3AM...it was an "after hours" club and the detector was going off like crazy...people kept shuffling in. Nobody stopped them to check. So it seems, whether you're sneaking something out or sneaking something in, this type of detector deters NO ONE! I guess I feel the same way about car alarms and virus protection. All useless tools that nobody pays attention to, so why do they exist?