Except these updates do fix faults. Fup3 for instance fixes the poi audio bug plus intermittent map errors, mbux freezing etc.These updates represent a murky area. The manufacturer is contractually required to repair faults. Where they discover a parts fault with batches of cars, they will either note the system as a recall, or if the remedy is a firmware update to the car that is viable with an OTA update, they will do that. I've had a couple of these over the two years I've had the car.
The so-called FU updates are not intended to remedy faults, they are largely cosmetic or other non-essential changes. That is why they do not appear as a flag on the system for the dealer to pick up. Since there is no clear fault with the vehicle, and since MB has not flagged it as a recall issue, the dealer will not be paid by MB for doing the work. As a consequence, they will either refuse to do the work, or will agree to do it, at a cost to the customer. Some of the larger dealers (I believe Marshall is one) have taken a commercial decision to do the work anyway. But not all dealers will.
I believe these updates are the tweaks MB was talking about at launch of the A Class in 2018, when they anticipated regular OTA updates over the first couple of years of the life of the model. For whatever reason, they have been unable to implement the updates OTA, hence the old-fashioned method of shipping DVD's to dealers. In true MB fashion, they have resisted the temptation to come clean to their customers about the reason for this farce.
They are certainly not just cosmetic changes, as tbh there are often not that many cosmetic changes made.
These bugs and errors are still faults, and as such are covered under warranty.
Hence why dealerships will update when you mention these faults with mbux.