@Sunshine It works pretty much like in FB. First you subscribe to a group, then you "post to the group" instead of sending simple messages.
I use Friendica group feature to follow Lemmy groups (this is how I got Rimu's message and this reply).
The point is, "native" friendica groups are not so common right now, the only big friendica groups I know are friendica's helpers, admin, devs groups. So, pretty much about friendica itself. You can find friendica native group directory here : dir.friendica.social/group
To have more, friendica groups one has to create groups and grow a community, people have to go there instead of FB, Reddit, Lemmy similar groups.
It's not a design flaw, friendica groups is just another offering for a federated platform group use case. The incentive to use native groups is all about developing a community around a particular topic, not much about the underlying technology I guess.