Apple added AVIF decode in 2022 (iOS 16, macOS Ventura) but never shipped an encoder, public or private. Photos cannot export AVIF, ImageIO cannot write it, and no other third party app on the store does it either. So if you wanted AVIF files coming off an iPhone, your options were "send the original somewhere else and re-encode it" or "wait."
On typical iPhone photos, AVIF lands around half the size of HEIC at matched visual quality. A 5 MB HEIC compresses to roughly 700 KB. HDR (including gain map HDR content from newer iPhones) is preserved, SDR works correctly across readers, everything runs on device.
The rest of the app does the usual library cleanup things: HEIC and HEVC recompression, duplicate detection, surfacing space hogs, swipe to delete, a private vault. AVIF is the new piece.