In the example, fnord77 posted, you can replace all the places that they used personal.github.com. It will be usable anywhere you would use ssh, including when going through a few tools that ultimately resolve to an ssh invocation. I won't copy the host block in .ssh/config but you could, for example:
`sshuttle -r my-cloud-server 0/0` for a poor man's vpn
`ssh some-enterprise-server` for when the server has a user unfriendly domain name that you don't want to bother remembering
`git clone git@github-personal:myusername/somerepo.git` when you want to clone using your personal key from GitHub.
There is the caveat, though, that you may have some nested dependencies that will use the plain ole every day host name in which case things will break for you. It rarely comes up, though, in my experience.
https://news.ycombinator.com/item?id=28242339