I imagine it's because the video series is introducing git itself, through Tower's UI.

git is a bunch of concepts: commits, branches, tags, a staging area, rebasing, pushing, pulling, the reflog, etc. Provided a git client implements these concepts they're a fine way to learn git.

Also note many (not you, just others I've met) 'I only use the command line' git users use git's inbuilt gitk GUI a bunch of the time. Sometimes it just helps to be able to see the branching in a real tree.