/content/usergenerated node is used to store User Generated Content, usually called UGC [1].
Consider a use case, where you allow website users to write comments on article pages. Now, these comments will be saved on one of the publishers. Using Reverse Replication [2] this comment is pushed back to the author under /content/usergenerated node.
You will have the option to initiate governance & moderation workflow on this comment, and once approved, publish this comment to all publishers.
[1] https://experienceleague.adobe.com/docs/experience-manager-64/communities/administer/user-ugc-manage...
[2] https://experienceleague.adobe.com/docs/experience-manager-64/deploying/configuring/replication.html...