I’m not familiar with the term unsers. However, there is a point for multiple communities. Lemmy is meant to be decentralized with no central authority, leading to multiple communities that are the same (or with similar topics) that have different rules and moderators.
As an example, the first one is based on posting AI generated images with no specific platform while the second one is to share tips, questions, and images created on Midjourney only. This shows the differences, but even if the topics were entirely the same, it would be important for choice and decentralization