Those testers apply a load of only a couple of watts to turn on PSUs that need their +5V and +3.3V lines loaded to run at all. They don't apply nearly enough to show how the PSU will perform under even a light realistic load, and I once had a PSU that worked OK when connected to a similar tester, but it would get very, very hot when connected to a real load because it had a bad capacitor that put a huge strain on a transistor. Besides, a PSU tester costs almost as much as a cheap digital multimeter and a pair of ten-ohm, 10W resistors but can't be used for nearly as much.