The old web let people lurk.
You could read a forum for six months without creating an account, learn the local jokes, discover who knew what, and decide whether you wanted to participate.
Many private groups reverse that order.
First you join.
Then you get to see the room.
Facebook’s current group controls show the distinction clearly. To join a private group, a user may need administrator approval. Hidden groups that a person does not already belong to do not appear in search results. See Facebook’s help page on joining groups.
For members, that can create a more comfortable space.
For casual observers, there may be almost nothing to evaluate.
Lurking used to be part of onboarding
Passive observation is easy to underestimate because it produces no posts, likes, or replies.
But lurking teaches people how a community behaves.
A newcomer can see whether questions receive useful answers, whether arguments become abusive, whether moderators are active, whether the subject really matches their interests, and whether the group seems worth their time.
If all of that becomes visible only after membership approval, the prospective member has to make the decision with much less information.
The group also loses the invisible audience that might eventually have become active participants.
Privacy changes the trade
There are strong reasons to prevent casual observation.
Support groups, neighborhood communities, professional spaces, parent groups, hobby markets, and communities exposed to harassment may not want strangers silently collecting everything members say.
Public readability can itself discourage participation.
So the relevant question is not whether every community should be open.
It is what each boundary buys and costs.
Some groups compromise by keeping rules, descriptions, FAQs, public announcements, or sample material outside the membership wall while reserving conversation for members. That lets outsiders understand the purpose of the space without exposing the people inside it.
Invisible audiences vanish from the measurement
For Dead Internet Theory, membership walls create another observation problem.
A researcher counting public posts sees nothing. A search engine indexes nothing. A casual web user sees a login or join button.
Behind it, thousands of people may be talking every day.
The empty-looking page is real.
So is the crowded room behind it.