$9.99 a month..

Why would an app like this require a subscription and not a one off payment.

I think many small developers are taking cue from large enterprises on how to price products without thinking it through. There was another similar discussion about a YC macOS notification widget company the other day.

The $10/mo is for a subscription to Setapp, which bundles a bunch of apps into one subscription service.

There has to be a reason for subscription, as in continued incremental value, cloud data storage and sharing, some sort of server backend that incurs monthly cost to run and so on.

Just slapping a subscription to a regular desktop application doesn't make sense.

Also, bundling is not a justification for a subscription, especially if one only cares about a single application. This is similar to the music albums which have already undergone unbundling and no one wants to see a bundling where there is no benefit to the consumer.