The point is that someone wanted to make it, and then made it. It doesn’t have to have billions of users to have been fun to make, or even to have reasons for existing. Pixelfed needs a few people to want to use it to keep existing. Instagram needs all of you + your data. And once it inevitably does something people are unhappy with, Pixelfed will be there. And once the service people will somehow find themselves on after Instagram does the same, Pixelfed will probably still be there, then. As long as one person is interested in running an instance/updating the software/either of those two, it will keep existing. People used to, and still do, post pics on their website. What’s the point of that? People who have friends on Instagram will stay on Instagram. People who have friends on Pixelfed/Fedi as a whole, will probably keep using Fedi. It’s not an all or nothing game.
If the community is hosted on the defederated instance then I believe you wouldn’t see that post either. I believe the community instance reposts the post so that it can federate to all subscribed instances. And those might only be known to the instance that hosts the community. So everyone goes through it as a proxy.