The only one that doesn't have a function easily discernible from its name is Miniature, which allows you to give your photos a cool depth-of-field blur. For instance, Monochrome allows you to create artistic black and white images, while Retouch lets you correct images by removing red-eye, healing blemishes, and getting rid of pesky extra objects. Each pretty much does as its moniker describes. Affinity Photo also boasts six extensions that can be used within the Photos app: Affinity Develop, Retouch, Liquify, Miniature, Monochrome, and Haze Removal. Within the app, you can do everything from editing RAW images to creating multi-layer compositions (and much, much more). Affinity is engineered to enhance the workflow of professional photographers, designers, and digital artists, but don't let that scare you away if you're interested - its interface and tools are intuitive enough for a beginner to use, making it super easy to amp up your images no matter your skill level. In fact, its iPad version actually won App of the Year back in 2017.
Like Pixelmator, Affinity Photo is a pro-level photo editor often praised for its sophistication and versatility.