Additionally, they allow designers to submit their own watch faces and share the revenue which I thought was cool. It helps combat the glut of "brand" knockoffs that fill many of the free/user-generated sites.
Originally, they offered a "pro" version that gave you access to all current and future watch faces for a subscription fee. I was not very keen on this because, like so many, I tend to forget to cancel these things when I stop using them. I mentioned this in an app review and so did many others. We requested a one-time IAP for the "Pro" version and soon after, the developer did just this.
So I paid $10 and every week or so I check out the new watch faces available for download. I like that some of the money goes to the platform and some to the designers. I like that the designs are high quality and don't include any knockoffs or blatant copyright infringement. And I like that I could just buy the full app after trying out the free options.
I've done the same with similar light-verion/paid-upgrade apps but this was the first one I'd run across that offered both a subscription or a one-time purchase.