Well, there is a point to be made, especially for Android, how poor the quality of the average app is and how they can often even be malicious.
As a user, I for example much prefer the harsh controls that Linux distributions have in place. If it's in the repository, you can essentially be sure that it does not do anything that's not probably actually in your interest, and just as well that it is actually decent software, somehow useful to someone. Probably the best software in its class, available on the platform, too.
Yeah, it also means that less is available on the platform. That maybe the highest quality software in a class is not always as high quality as in other distribution platforms, but for most jobs really, you don't need the non-plus-ultra, you need a tool that does the job and one of which you know that it does the job, without trying it out for hours or having to fight defaults that work against you.
On Android, F-Droid sort of provides that, but it's still just a small island of decency glued on top of a system and ecosystem that lives off of user-hostile behaviour. If as a user you don't know of F-Droid, you're hardly going to find these decent apps in that sea of not-so-decent apps, which really discourages the creation of decent apps as well.
With a curated platform, you could provide the user with trust in their platform (which might especially also be sellable to businesses) and you can keep the air free around those actually decent apps.