The legitimate uses feel kind of rare. Maybe there's some stock photo abandonware out there (questionable "legitimacy", but it's not so out there)? Maybe someone bought stock photos from a company that went bankrupt and never downloaded the non-watermarked version, and somehow that company's IP isn't accessible now? Feels like a stretch.
Upscaling old purchased images feels like a more common need.
Incorrect. Removing the watermark constitutes a derivative work. To distribute this work you need permission from the copyright owner to be legal. This you will almost certainly not get since the point of watermarks is to keep people from stealing copyrighted material.
You have a picture of yourself or a friend, with a watermark on it. You remove the watermark. Now the picture looks nicer when you look at it. Why's this difficult?
It's difficult because people are trying to figure out where you obtained this hypothetical watermarked image from (that you can't get a non-watermarked version of) and what you plan on doing with it after besides looking at it yourself, and how this becomes something for sale to "enterprise-level clients".
From their website:
> The features your business needs to eliminate watermarks for good.
> Whether you're a photographer or a social media manager, let our AI handle the cleanup so you can focus on creativity.
They seem to think that this is a need businesses have.
Upscaling old purchased images feels like a more common need.