DISCLAIMER: Obviously some git tasks are just the wrong fit for a GUI, and some git tasks are just not possible.
I've not used Fork but I find SourceTree (which looks very similar) MUCH quicker to search for, and build commits from, hunks. I mean perhaps if I really invested in the git tool I could get my speed up, but I don't see the point - I have what I need already in SourceTree.
More generally the tree-structure of the git filesystem often lends itself to persistent visualisations e.g. split view graph + branches. Especially when fetching, and the graph automatically re-renders and shows you an unexpected structure.
If we are talking about vanilla git - GUIs can be a nice drop in to speed up commit work flows like "checkout that commit I was working approx a dozen commits ago". Being able visually-grep, and then double-click is a bit faster than `git log --pretty=oneline; git checkout SHA"`
And finally, and they are a great on-boarding wrapper for users who are new to git. I've had good success unblocking users with very little git experience who are only using a git CLI, by introducing a git GUI. This really flattens out the learning curve, which frees up both devs to concentrate on the _actual_ task at hand.
