Fair point. The way I see it - if you think about beauty, you must already have accomplished everything else. Otherwise yes, beauty for the sake of it is useless.
Say I have a knife - of course I want it first and foremost to be a "good" knife (again, "good" between quotes, because that is also subjective). But for a good knife that I'll have around for many years I want it to be also beautiful. I would not want a good knife with an ugly plastic handle. But of course, like you say, I would also not want a beautifully carved knife that is unusable.
This Umbrel thing we are discussing here - I happen to love it, and I've been using it constantly for more than half a year. I looked at the competition as well, and guess what - the alternatives with ugly UIs have a terrible architecture behind the scenes. Take RaspiBlitz for example, the most beloved competitor of Umbrel - I went through the source code trying to understand how it works and I was horrified. All just files full of magic commands placed in magic locations. Good luck trying to deploy an app you create on RaspiBlitz! Sure you can, but it's not "nice". Umbrel on the other hand has a clean architecture and a short tutorial on how to make any app run on it - dockerize it, create a yaml to describe it, BAM!
Never tried the Start9 Embassy, but I think it's at least equally good, and maybe a little bit less beautiful. I'll try it anyway. But I don't see myself trying the outright ugly alternatives.
I guess somebody that tries to make something beautiful also tends to find beautiful solutions to otherwise mundane problems.