What would be the point of embedding Redis into an application? What's the advantage of using Redis over using the builtin (or third party) data structures of the language the application is developed in?

I'm asking as a non-webdev who never quite got what Redis actually does, but would love to learn.

Probably because Redis gives you a very well-defined/understood set of rich data structures with built-in behavior like TTL, atomic operations, eviction, and persistence. These things are otherwise usually scattered across native types, helper classes, or entirely separate libraries.

In practice, mostly scaling sessions and ephemeral data (caching) across multiple intances of a microservice on multiple machines. Seperating the kv store and the application allows upgrading each application while retaining availability and avoiding loss of session data.

For simple cases, it is probably a total overkill to even consider it, but for something heavier, embedding the database gives you a chance to trivially migrate later to a separate database server.

Why would you embed SQLite?

It’s the same use case with a different api.

A typical (meaningful) example might be communication between threads or actors in a single process, or idempotent tests.

As with SQLite, an external xxx that does this for you is certainly better, etc. but it’s convenient sometimes, to have an application that doesn’t go “now before you run this install Postgres…”.

It’s seldom useful for a web app where you control everything.