Because a significant number of people still interact with the desktop via mouse rather than keyboard shortcuts.
Hell, I use hot keys for most things but I still often prefer to quickly minimize a window with the cursor instead of reaching across the keyboard. The first thing I do with a vanilla Gnome installation is get Tweaks on there and restore window buttons.
Also PairDrop. I tested a few of these sharing apps and found this one to be slightly better for reasons that I can no longer remember.