They may or may not be a wast of money. It depends.
One can have a very high quality dedicated AC mains line to the audio gear and high quality outlets, plugs and power cords.
But there can be electrical/electronic devices that produce unwanted electrical noise and impress this noise on the AC main power.
Below is a generic example of what might be seen.
This is much like the reception of FM stereo signals, each location can be different.
In a perfect world the internal power supply of the audio devices would
filter out all of the unwanted electrical noise, but this is not always the case. Unwanted common mode noise can be notoriously fickle in it ability to go where we do not want it to go.
Below is an example of a basic 2 stage passive device.
Adding things in line with the AC mains power can impact the dynamics of the music because of I^2*R losses and so on. This has been much discussed on AK.
Power conditioning can range from simple devices as above to full blown power regeneration.