To allow users to make their own groups, with the members viewing and participating in the group OF THEIR CHOICE.
Sample public post. Anyone may comment on a public post in an Organic Group having public posts
There may (at the same time) be "private" posts that can be seen only by registered users who JOIN the group _OR_ registered users allowed (by some other criteria) by the group owners can decide who the members can be.
(This present Organic Group happens to now have one post that can be read by members only, in addition to this present post.)