It's traditional software, it runs offline, client side. Which is great for low bandwidth places and the privacy concious. It has some basic ML for face recog. The money is in photo printing, which is a pretty big industry which Google has failed to capture any of.

It had a huge installed base, but of course Google would not sell user engagement in bulk, only in tiny pieces as ads. Advertising the things people take photos of is pretty straightforward too, another reason why Google killed Picasa instead of letting it free.