A good use case for Discourse as the SSO provider for WordPress would be to allow a forum to use WordPress plugins to extend Discourse. For example, if a forum needs to allow its users to sign up for events, it might be easier to link to a WordPress site that is using an existing Events plugin than to write and maintain an events plugin for Discourse.
Possibly, a WordPress multi-site installation could be setup to serve multiple Discourse forums with a selection of plugins that deal with users. The kind of plugins I’m thinking of would deal with things like appointment booking, event registration, mapping users, buying and selling…
This could also be done with having WordPress as the SSO provider for Discourse, but as you say, if the forum is the main reason to have users login, then it makes more sense to authenticate through Discourse. Especially for an existing forum, it would be an inconvenience to users to force them to login through WordPress if their main reason for logging in is to get to the forum.