I suppose, then, it's for those who don't want to be tracked, and not a "very stupid idea" per se.

But then again, if you don't trust the remote to know who you are, then why do you have an identity with them? I mean, the remote service is SUPPOSED to know who you are. That's kinda the point.

I don't think so.

GitHub should know I'm the user who has access to push to repos a, b, and c.

AWS should know I'm the user who has access to update code or data at places d, e, and f.

But neither needs to know my full identity, or about each other, at all.

Right. What about paid service. Shall I pay with someone else's CC or ask the bank for another one just because I don't want to be tracked.

Many banks and credit card companies offer a service that lets you generate a one-time-use credit card number.

But your billing address will still be the same

So what?

Paid services necessarily require a higher level of trust (since you are handing them money) than random internet services. So we are off-topic from ssh keys and identity.

If you don't want someone knowing your personal payment details (CC #, billing address), then pay in cash and use services don't deliver things to your home. And if you can't, then just don't use a service.

But that's living in way too much paranoia for most of us.