Sometimes you have tools that reference a github url with https and you want them to use ssh instead so your ssh key works.
In my case it is Cocoapods trying to clone a spec repo. Since I just enabled two-factor auth on my account it now stopped working:
Matts-iMac:Nutrition matt$ pod install
Cloning spec repo `enquos` from `https://github.com/enquos/mib.ios.pod.podspec`
Username for'https://github.com': hammertoe
[!] Unable to add a source with url `https://github.com/enquos/mib.ios.pod.podspec` named `enquos`.
You can try adding it manually in `~/.cocoapods/repos` or via `pod repo add`.
By putting the following in your ~/.gitconfig file you can force git to use SSH whenever it is asked to use a HTTPS url: