Sorry, this was a bit out of context. As the other commenter assumed, it is for replacing/extending the functionality of the phone. For example, I wanted to access the heartrate and other mi band data and it's quite easy with gadgetbridge and my own flutter app.

I tried similar apps before with SwiftUI and the Apple Watch and it was pretty much a disaster (although I have working knowledge of objective C ...). Dart came easy and flutter seems super powerful to me ( I just re-implemented my first very simple ios game in just a day. that's the objective c version https://apps.apple.com/us/app/blanks/id286883373 ). Will publish/open-source the other one, when it's polished.