Usecase: you need to manage multiple git accounts (e.g. private and work), with each their own SSH key.
Example: you use host Bitbucket privately, and your firm starts using Bitbucket as well. Or client B has set up an account for you, which uses the same host you’ve already used.
Fabian Keller has a simple way of working with this. Check out https://www.fabian-keller.de/blog/configuring-a-different-ssh-key-per-git-repository/