While I can’t speak to specific apps alot of times it’s house cleaning stuff.

Maybe some bug that affects a certain number of users is found and fixed. And the update resolves that bit, since you weren’t affect, you don’t notice it.

Other times it’s to include fixes in libraries they’re using. So, for example, a JSON parsing library may have a security fix and they updated their app to use that newer version.

Another could be some behind the scenes api/library updates. Maybe a service they’re using for content (such as interacting with Lemmy) or maps or advertisements is being updated and they need to point their app to the new service address or change how they interact with it.

And of course there could be feature updates but those, usually, would be things you’d notice. Although, in some cases, it may be packaged with the application but waiting for some criteria (a backend service to be ready) or may even be part of A/B testing where some users get one change while others don’t so the developer can see which features are preferred using real data.