A cache can help even for small stuff if there's something time-consuming to do on a small server.

Redis/valkey is definitely overkill though. A slightly modified memcached config (only so it accepts larger keys; server responses larger than 1MB aren't always avoidable) is a far simpler solution that provides 99% of what you need in practice. Unlike redis/valkey, it's also explicitly a volatile cache that can't do persistence, meaning you are disincentivized from bad software design patterns where the cache becomes state your application assumes any level of consistency of (including it's existence). If you aren't serving millions of users, stateful cache is a pattern best avoided.

DB caches aren't very good mostly because of speed; they have to read from the filesystem (and have network overhead), while a cache reads from memory and can often just live on the same server as the rest of the service.