As other people have said, they don't do a lot - basically the popular ones, as always, are just those that are marketed the best, or lied about the most.
We sell one that works as well as any, and it doesn't cost 50 quid, it costs what it's worth
https://www.lencarta.com/flashgun-mini-diffuser-ball
Basically, these things only work indoors, they are a waste of both time and flash power when used outdoors. Used indoors, some light reaches the subject directly, and is very slightly softer than if it wasn't there, because it's very slightly bigger than the flashgun, and it's size that matters. But, much of the light also travels upwards, and bounces off of the ceiling, which makes the ceiling the light source. This really does make the light softer, because the ceiling is big.
And some of the light bounces sideways where it will reflect off of the walls, if they are close enough, and again this makes the light bigger.
Be aware though that all of this is just a quick fix, and you'll always get more consistent and better results if you can take the time and use the grey cells to plan the lighting.