That is a value for the entire gpu, what about the memory part itself? Also consumers don't need 300GB of it (yet).

But to answer - memory is progressing very slowly. DDR4 to DDR5 was not even a meaningful jump. Even PCIe SSDs are slowly catching up to it which is both funny and sad.

As for the usecase - I use my memory as a cache for everything. Every system in the last 15-20 years I used I maxed out memory on, I never cared much about speed of my storage, because after loading everything into RAM, the system and apps feel a lot more responsive. The difference on older systems with HDDs were especially noticeable, but even on an SSDs, things have not improved much due to latencies. Of course using any webapp connecting to the network will negate any benefits of this, but it makes a difference with desktop apps. These days I even have enough memory to be able to run local test VMs so I don't need to use server resources.