You don't have a lemmy account, but kbin gets content from lemmy pushed to it. So you can subscribe to subs that are on Lemmy. That doesn't mean you can't sub to the kbin equivalent; in due time, both may have a memes sub. But if you prefer lemmy over the kbin, you can see and interact with that lemmy content from here on kbin. You don't need to be on Lemmy to do it, the instances are federated.
Likewise, say in the future a sports-only instance is created. It exists solely for sports headlines across all the leagues of the world. It is way better than content on kbin. If kbin federates with that group, you can see and interact with that content.
Say there is a group that doesn't meet your instances ideals. They like to be inflammatory and provocative because they can. Your instance can not federate with them, so you won't see that content when viewing the All section of your instance. Likewise, they won't be able to interact with content you post. This prevents brigading and intermingling of groups that would only argue and conflict.
You can enjoy all the content of any federated instance from whatever platform you want.