Fans need to be plugged, preferable, to the motherboard fan headers. Then, the mobo will control the fan speed/temps in the PC case. If you plug the fan into a fan-to-molex adapter to the PSU, then the fan will run at top speed all the time.
There are still some devices that still use them (i.e., front panel USB expansion adapters, some sound cards, some SATA add-on adapters, etc). They are becoming more uncommon.