When I'm explaining KV to a junior dev, I do it something like this:

Memcached got big during a confluence of two problems that have both been solved now. First, we had a bunch of big, semi-stateful web applications built in programming languages with GC, and server memory got bigger than you could reliably collect without long pauses. Pushing some of your data into a separate memory space raises the ceiling considerably.

Secondly, during this ten year window, ethernet cards were half an order of magnitude faster than hard drives. Putting stuff on another machine could be faster than sending it to swap, a memory mapped file, or some more sophisticated data store (like a database).

We don't have to struggle with these now, and half the time we avoid the first one altogether. They still have lots of places they are used, but you are way better off working to cache inbound requests instead of outbound requests. That lets you move a bunch of caching to the edge of your network, or to the user agent.