Probably the main value-add of StGit versus the git-only workflow described is that StGit makes operations on its stack of patches fast and fluid.

Reordering patches with StGit is trivial with `stg push` and `stg pop`. Arbitrary patches can be combined with `stg squash`. A modification in the work tree can be incorporated into the top-most patch with `stg refresh` or an arbitrary patch in the stack with `stg refresh --patch <patchname>`.

In other words, StGit provides a command vocabulary for performing these kind of patch-stack manipulations in a first-class manner.

But as you note, everything StGit does can be accomplished with plain git--albeit with more or less difficulty depending on the particular stack manipulation goal.