I've read that most of the time these things happen within seconds, so they must know which bags can be opened fast, or even what bags are likely to have something good in them. I would think they would be mostly looking for jewelry, but anything that can be sold cheap works too. Some airports are worse than others, although I don't know if the actual stats are available.
At the end of the day its like a club on a car, yes it won't stop a determined thief. but it will stop the joyrider. Maybe adding a luggage strap is just enough to slow them down to not bother in a lot of cases, but might also mean that if they get a minute alone with the bag, it will flag it as having valuable in it?