Multiple compilation targets require increased implementation complexity so you would think they would have a good reason for when you would compile to one target or the other. I couldn't find any information about it on their website, so I was hoping you would actually respond with something helpful rather than... that.
To answer your question about why you would use Gleam when you're running code in a JS engine, it's the same about any language that compiles down to another, such as Clojure, and you can take your pick of those reasons: preference, syntax, pragmatism, etc. I'm not sure what kind of answer you're looking for beyond that.
None of your reasons ("preference, syntax, pragmatism") fully explain the situation without more context. Whose preference? What syntax are users even seeing? Why is it pragmatic?
Compilation targets are generally chosen based on performance characteristics and where the code is actually expected to be run. If you're not expecting the code to run on the client, then there's no reason to use JavaScript. V8 performance is better when single-threaded, but then why use the actor model?
When I go to the ClojureScript website, they have a whole section about why they chose JavaScript as a compilation target[0]. Notably, it mentions the word "client" multiple times.
It seems your stuck on js-only for ui and somehow rejecting compile to js. If you don't wanna use it fine but I'm not sure what you're looking for in this thread...