Because everyone wants their preferred set of tools/languages to do it using the design patterns they like and there is nothing wrong with that. I don't see myself using Beeware ever really but that doesn't mean people shouldn't work on it.
the last line of the comment you replied to gave a pretty good example: If you have tools build in tech X and just want to wrap them in a UI to make them usable on a phone. (similar to how Python ships with Tk bindings for UI: no, its not pretty, but it's an UI and thats what you want sometimes, without learning something big or worse a different tech stack)
https://play.google.com/store/apps/details?id=com.keith_mage...
Also a note re their platform compatibility, it's not just Android and iOS: iOS, Android, Windows, MacOS, Linux, Web, and tvOS
Looks very interesting, going to give it a go adding guis to some existing cli python toys I've built