1) The ability to conveniently import data. I have years of weight data that I would like to import, but I can find no way to do it without writing my own software.
2) The ability to conveniently export data. Google Takeout has no listing for Fit. Of all the types of data in my Google account, health data is the most likely to be useful to me elsewhere. Therefore, I will continue to use a spreadsheet as my system of record, and Fit only as a possible throwaway.