targeting ads is only one teeny tiny use case for user data. Building general AI is a race and the more user data you have the faster your AI will learn. You definitely want to limit the amount of data your competitors have.
It's the biggest one, followed by company specific recommendation engines.
Usage stats could help you improve a user interface, teach users new skills (eg: "you always do this, here's a shortcut to save you time"), perhaps more personalized operating system interfaces and defaults.
You probably don't need a fancy pseudo "AI" system for any of these, but that's the current gold rush so someone's gonna do it