They heat (or warm) the hand or shower towel..... (the name says it all). Otherwise, you could probably do without these things (ours never run - I wouldn't buy them again).
Ok, but I think I'd rather have a thing like that than an ugly standard radiator in the bathroom, right?
Yes, certainly..... But we have underfloor heating (ok, it runs in winter) - that gets warm enough and every room can be controlled individually. In summer you simply don't need a towel warmer. And honestly, not in winter either.
I have one towel radiator in each bathroom so that the 24° in the bathrooms are met according to calculations. I always run them at level 5 and was able to lower the heating flow temperature. Of course, the towels do not get warm at a flow temperature of 25-33° (depending on the outside temperature).