Better security, for once. You get (security) updates a lot faster with GrapheneOS.

Also on GrapheneOS, Play Services and Play Store come unprivileged, sandboxed like any other app. So Google is not an admin on your phone, which I would argue is one step towards "severing the umbilical cord".

Moreover, GrapheneOS doesn't have any issue with apps sideloading.

And more. There are many reasons to use GrapheneOS.