Comment by ItsBob

It's for your login and payments. I need to verify that you are authenticated somehow and Google/Apple also handle payments.

You "Login with Apple" or "Login with Google". They manage the login entirely and pass me your id and an access token (assuming you pass their login test). I store that in my DB so that your data from the app can sync (the paid-for app syncs your training data to my backend but I match it only based on the Google/Apple id.)

The alternative is that I build my own auth system and I'd need to store something you can type in the next time, e.g. email/password address etc.

If you have an Android/Apple phone you're already authenticated with them. I just need Google/Apple to say "this guy is cool, let him in" and I then use the id to check if you've paid, sync your training data etc.

On its own, the id is useless! Means nothing and cannot be traced back to a person. I genuinely do not know your name, email, what country you come from, GPS data, CC data. Nothing at all!

I don't want your data.