That is one use case. There are several reasons to run a mailing list and a community site with various levels of overlap.
For instance, this site is our support community. 
We might want to send out a newsletter to people who are not using our services or Discourse, because we also operate in other spaces online. 
In such a case, we could make it easy for our forum members to join our mailing list, even if that isn’t the main purpose for it.
Sounds like your use case is covered, no need for a newsletter plugin. 