It says on the page that
As we’re committed not to add paywalls, this purchase will not grant you any additional features in Immich. We rely on users like you to support Immich’s ongoing development.
So this is a purchase which grants you no extra bonus, which is functionally a donation. This is a common practice with software supported by Futo - they offer the software for free, but ask that you pay a good-faith “license fee” for the software anyway to help fund development. I think Louis Rossman has a video about it somewhere, if I remember I’ll link it in an edit when I have time to find it later.
Dang if only you could click the “about” link at the top of the page to find a comprehensive explaination of the software…