A refresh token is an engineering tradeoff. Without refresh tokens, you would have two unappetizing alternatives:

• an access token that lived for a long time. In this case, if the access token is stolen, the attacker has a lot of time to access systems (or you need to have some kind of access token revocation strategy, which degrades the value of stateless access tokens).
• requiring the user to sign in every time the token expires. That gets old if the lifetime of the access token is minutes or hours. I even get annoyed every time Google asks me to re-sign into gmail, which only happens every week or two.

The spec requires a client to explicitly request a refresh token. With FusionAuth you have to request the offline_access scope (which is common for other auth providers, but I wasn't able to find it in the RFC), so it's a way to offer more flexibility.