When has to be used a ferrite bead in power supply?
A ferrite bead adds no resistance at DC but it designed to be lossy at (usually fairly high) AC frequencies.  That means it is truly resistive-like not purely inductive for high frequencies.  So the reason the bead is there is to provide a higher impedance to noise in the RF range.  Beads are often added when there is an EMI problem, or when there's circuitry that is highly sensitive to high frequency noise.

To pick a bead you have to know the approximate frequency spectrum of your noise.  Then you look at the bead datasheets to find one that has enough AC resistance in that frequency range to solve or mitigate your problem.