Ex-app developer here. We do it because reminder notifications boost our re-engagement by over 10%.
When an average user downloads an app, there's like a 70% chance they'll use it day 1, a 10% chance they'll use it day 7, and 1% chance they'll still be using it by day 30. A simple reminder notification after day 3 or 15 can drastically boost those numbers.
Why do we care about the numbers? Because Google and Apple care. They see higher numbers, assume it's a good app, and make it show up in the search results more frequently. This gives us more downloads.
If you're putting the time in to craft a quality app, you probably need money, which comes from ads and subscriptions, which is funneled by the number of downloads. If you don't like that as a user, stick to FDroid.
My experience working support for a phone manufacturer has informed me that once an average user installs an app it tends to stay installed indefinitely, but they may or may not be aware it's even installed. A gentle nudge notification of "hey look at me" every once in a while might very well be amazing for engagement
Or maybe, and I know this sounds utterly insane but hear me out:
Instead of reminding people to use it with notifications you could use the memory of previous engagements to make the user actually initiate subsequent utilization of the app. It's kind of like the user notificating itself.
It misses the point. As an app developer you have KPIs. You want to hit those KPIs. Adding these notifications helps do that. Obviously most developers will do what you're suggesting, but that doesn't mean they won't add notifications