When to use a key-value store for web development?

When would someone use a key-value (Redis, memcache, etc) store for web development? An actual use case would be most helpful.

My confusion is that a simple database seems so much more functional because, to my understanding, it can do everything a key-value store can do PLUS it also allows you to do filtering/querying. Meaning, to my understanding, you can NOT do filter like:

select * homes where price > 100000

with a key-value store.

Example

Let's pretend that StackOverflow uses a key-value store (memcache, redis, etc).

How would a key-value store help benefit Stackoverflow hosting needs?