SourceTree requires that one and only one SSH key can be configured. For those of us who use multiple external system (e.g. github.com, gitlab.com, self hosted sever, etc), a typical security practice is to use different SSH keys for each system, so that if anyone key was compromised, only one system is potentially affected. OpenSSH supports this quite easily by setting up the config to use different SSH keys based on host, e.g.

Host gitlab.com
  HostName gitlab.com
  IdentityFile ~/.ssh/id_rsa_gitlab

Host github.com
  HostName github.com
  IdentityFile ~/.ssh/id_rsa_github

This works with git on the command line, but not in SourceTree.