IMO it's simple: with undo you want to 'go back in time' so you want to remove the commit and get the exact same list of staged/modified files that you had befo

IMO it's simple: with undo you want to 'go back in time' so you want to remove the commit and get the exact same list of staged/modified files that you had before (especially useful when you made a git commit -a which added too many files in the commit)