There's a third use case: multiple identities at work.

In my case, company I worked for got acquired by a larger corp. Things happening as they usually do, I ended up having two different e-mails/identities/SSO credentials - me@old.company and me@new.corp. Most of the code I worked on was stuck on old company's infra, but new repos were developed on the acquiring corp's infra, so for years, I had to maintain two different SSH / Git identities too, and use appropriate one for a given repo.