this is the ad-free version, which is available with the exact same (if I’m correct) features on F-Droid for free, along with the source code on GitHub.

the versions on the Play Store (paid version and free version with ads) likely just help pay the developer for their work

and as others have said already, free software is free as in freedom, not free beer.

it’s free on f-droid, you can buy it on play to support the developer