what if the service is a way to manage your data? In my case its a photo library alternative for fitness photos/videos. Take a picture of your handstand every month and store it locally in the app documents folder or on PhotoKit so that you can see improvements over time (nothing is stored in the cloud). I don't know of a way to do that on a mobile website.

I wish you well.

I wont be a customer because I don’t care enough about my looks or fitness to get into daily photos. But, really, what does your app offer over the photo gallery and folders already on my phone?

I personally like keeping it separate from my phone's photos/video folders. You can also add notes/rep counts and add links to useful workouts you find online.

What personal data (that you didn't explicitly provide or allow access to) does an app have access to that a website doesn't?

There's a million things they can do. Permissions aren't that fine-grained, and the apps always have ways to infer a lot of stuff. Heck, they can even make background tasks, while if it's a website I know that if I close the tab your tool is gone.

There's a whole range of things that an app can do/infer on a system that a web page in a browser can't.

Things like processes that are running, RAM, battery life, and more.