It lets you charge a smaller amount up front, to get more people trying, and paying for your software, but keeps an ongoing revenue stream to keep the business going as well.
We would prefer the traditional model of "pay for major feature updates every years" but the app store does not support that.
Sometimes the simplest app having a subscription seems like pure greed from the developer. The excuses are generally that of increase in development costs. But maybe that is equivalent to increasing the cost of development like increasing the cost of living when there is extra money coming in.
The only recurring charge I pay for is Amazon Prime, because I don't want to think about shipping costs and I like Prime Video (replaces Netflix) and Fresh (free grocery delivery, no need for Instacart)
Other than that, anything that requires a subscription, I can do without.
I've bought one mobile app in my life: Backcountry Navigator. It lets me download detailed maps for hiking and then use them where there's no cellphone towers. The free version was good enough, but it was so useful that it felt better to pay the developer his $7 for lifetime usage (no recurring charge)
But I think the sentence you're quoting is referring to the following: BC Navigator allows you to download maps for offline use from a variety of sources. Most of these sources are free (e.g. Open Cycle Map [related to OpenStreetMap], US Forest Service, etc) and some require a paid account with the map provider (e.g. CalTopo). So if you want to use the CalTopo maps, you need to pay CalTopo. But it's perfectly fine with the free maps.
The UI used to be pretty difficult to figure out. After a year I got used to it and it's fine. But they released a new version for iPhone which is supposedly quite different from the early Android version, and they claim they put more effort into making it easier to use.
Another thing that's very useful about BC Navigator is that you can easily import GPX tracks (e.g. planned routes) and overlay them on the map in different colors. So you can have your main route for the day, and maybe some alternate routes for a section where you're not sure if the original route is going to work out. You can keep an entire library of routes in the app, and select which ones you want to show that day.
It's just a perfect app for what I love to do - mountain biking in places where few people go.
To be more specific https://www.opencyclemap.org/ is generated from OpenStreetMap data and some terrain data, likely SRTM.
I'm happy that I can get by without paying for monthly subscriptions. I am also happy if there are lots of other people who find enough value in these services to make it worth paying for.