cosf you can use the tokens to interact with Flarum's REST API with the user the token belongs to. Basically it lets you do anything you would do using Flarum web interface in a script. It doesn't give your user any new power. Everything was already possible using short-lived cookie session tokens before, developer tokens just have the benefit of not expiring after a few hours of inactivity.
Access Tokens are described in our documentation https://docs.flarum.org/rest-api#access-tokens
The documentation still says developer access tokens cannot be manually created. We need to update it now that the token management UI has been added.