Who asked for this, and why?

It exists to reduce dependence on YouTube.

There are bad things to say about Google all day, yet consumers will still line right back up to use Google's video service without fail. Having that content mixed in with other sites reduces dependence on YouTube over time, as creators consider decentralizing their media presence and posting content on Patreon, Nebula etc, instead of Youtube's walled garden of Communities, Memberships and of course the channel itself.

IMO right now this app is really for people who want to support OR already are supporting creators, and are displeased with the amount of apps, bad UX and poor integration this experience entails when done outside of Youtube's platform

If that's not for you, then maybe the privacy aspect of Grayjay is of interest, where the YouTube integration allows you to control what data is associated with your Google account. Aside from those, most people should probably stick with their existing YouTube client. This app is targeting a specific kind of audience, and fits right in with Rossmann's