They come and go. They're random clutter. We only need a few big instances that hosts a majority of the communities and that's it. Why do we need so many smaller ones?
I think it's a valid thing to consider. Due to the nature of the fediverse, there will always be small instances. I guess you could say their purpose is to allow Lemmy to be decentralized.
Small instances don't prevent Lemmy from being centralized, people don't mind if their instance defederates from an instance with 5 people. Medium-sized instances (e.g. dbzer0) prevent wrongful defederation, because people will suddenly miss a few communities / users.