It's for paid projects. So you can give download keys to your patreons without doing it manually.
If you have the same files on the Itch project as would be accessible in your patreon, that feature is redundant, yeah.
On the other hand, what about people that cancel their patreon subscription. If they have claimed a download key, they will still have that download key, I would assume.
There probably are use cases for this, where this integration is helpful. Maybe someone can explain a use case here.