Devs aren’t too interested in how good or bad the system UI looks though, I assume. Surely some are because they want to have their apps look as „default“ as possible, like WhatsApp or Navigator (my public transit app of choice), but I believe most get a dev beta, especially an early one like this, to see how their apps tie into the system, their share sheet options, their control center toggles, their widgets, their notifications and so on.
But they can only really do that if the design is mostly finished.
Again, I’m assuming here.
Yes, Apple probably wants feedback on the design (through their official channels) from devs, but if the design is THE main new feature, Apple themselves should have figured that part out before giving any portion of the public access to it.
The dev beta is there for devs to make their apps work, the public beta is there for the „general public“ to point out any rough edges, that’s also why the dev beta releases a day before the public beta, except for when a new major version drops.