The point is to be one front end to "[a]n open platform that lets users control their fitness data" and "lets developers build smarter apps and manufacturers focus on creating amazing devices." [1]
> You can manually add activities but the only options are: Walking, Running, Biking, Other.
The backend is aware of more activity types [2], so I suspect its either getting the right UX for that is the issue, or that developing the what to do with other activities to make the data meaningful that is holding back putting more into the app UI.
[1] https://developers.google.com/fit/ -- the page that, for me, comes up as the first web (not news) result for a Google search for "Google Fit".
[2] https://developers.google.com/fit/rest/v1/reference/activity...