The permeability of the ferrite bead increases the inductance of the wire going through it. This increased inductance increases the impedance of the wire at higher frequencies.
The increased impedance reduces the amount of RF common mode power that can flow in either direction:
reducing harmonics from a power supply's DC-DC converter switching frequency from entering the device as RF noise,
and reducing any digital RF noise created by the device from using the power supply cord (and building's AC wiring) as an antenna that can radiate EMI into the environment (potentially exceeding EMC regulations, such as CFR 47 Part 15 in the U.S.)
Both of the above are especially important with any radio equipment in the room, where one want to reduce the RF noise from interfering with signals of interest. (radiated and conducted EMI, etc.)
To hear this RF noise, take an old analog AM radio, tune it between stations, and hold it next a cheap LED light or USB power supply. You often will hear a very nasty buzzing. You might want to reduce the amount of this power supply noise that gets conducted into a radio receiver or other circuits or equipment that might be sensitive to RF noise.
A poorly filtered power supply can increase a radio's RF noise floor and completely obliterate amateur radio weak DX signals. Often a single ferrite bead is not enough, so one might add multiple clamp-on ferrite beads to the cord(s), or wind multiple turns of the cord(s) through a ferrite toroid of a proper mix to increase the impedance at an RF frequency range of interest. Amateur radio operators are known to do this, not only for power supply cords, but for every network, USB, and video cable in the room.