This seems weird to be explaining to anyone in 2022, but there's lots of interface use cases where the trackpad isn't a very good solution. Thinks like graphic design/layout work, spreadsheets, filling out forms where the UI is tight. There's also the matter of a physical mouse being MUCH, MUCH, MUCH better for people with loss of hand dexterity.

All that said use cases for this device are pretty rare because they mainly focus on people who need to travel between locations to work frequently, but also require a mouse. This seems great if you are traveling with a small laptop or tablet in a small sleeve. Fitting this into that package would work great, and a mouse is like carrying a stone of about that size (basically requires a different kind of bag). I work on a full-sized laptop, so carrying a mouse around isn't that big a deal.