After you create a new branch you are taken to a confirmation page that says "Checkout your branch."
There is a button "Checkout in SourceTree"
and below there is some text formatted as code, a command line code for fetching and checking out the new branch.
git fetch && git checkout ABC123-branch-description-summary
This line is helpful, but if you run it you will fetch every branch in the repo. This is not necessarily desired.
Why not this?
git fetch origin ABC123-branch-description-summary && git checkout ABC123-branch-description-summary