Product tiers come naturally with IC's, by disabling physical sections of the chip that can't work due to inevitable manufacturing defects. You don't throw defective chips away, you physically cut off those defective sections of the chip, with a saw or with a fuse, so you can sell it without that defect. This works great in the beginning, when your yields are low and you're still dialing in the manufacturing process. But, as yields increase, you might not have enough of the lower tier chips to meet demand. So you, just fuse otherwise good chips. Chips are cheap, after all!
There was probably some cost/engineering justification for it, but there's a very high chance that justification is looong gone. Chips are cheap, but revisions are expensive!
Not really.
If the fuses are for security, sure. But these aren't.
These fuses are for economic extraction. Doing it the way NVIDIA did it is a safety feature: it ensures that blowing a fuse will never destroy economic value -- it will never turn a card that was previously able to do something correctly into a card that can no longer do that thing correctly.