Boosting is super important in all contexts in the Fediverse.
When am instance subscribes to a content source - be that a user actor or a group actor - on behalf of a user, it only requests future content. Back catalogues are not fetched by default. Boosting re-publishes the content, so that it is received by new followers.
With a group actor, the boost triggers the actor to reboot the content itself, sending it out to new subscribers to the group, and filling in that back catalogue.