If the communities could federate, then posting to /c/rust@lemmy.ml will not automatically repost it to the other 2, instead subscribers to one of the other 2 will just also be able to see the post. I.e. it eliminates the need to subscribe to all 3, and thus also the need to post to all 3. That's how I understand it at least.

If I understand it correctly it's basically community to community federation, so when you subscribe to a community it also pushes other community posts it follows to your subscribed feed. Those posts would still show up as those other communities. A community becomes not just a place to post, but also as a curator that vouches for other communities as well. You could even potentially have a curate only community like a lemmy best community list to get newbies started, or category level communities.