Who performs an audit of your ERB and IPERMs, and when would this happen?

First of all, you should review your own record at least annually, and whenever you have a document to add.
What you are hearing about is probably the "triple A audit"; the Army Audit Agency (and a contractor) performs audits of iPERMS for pay-related records (promotion orders, 5960, bonus addendums, marriage cert and dep birth certs, special pay orders, etc).
If a document required for pay received is missing, the Soldier receives a notice to provide the document within 60 days or face a collection.
I have never "seen" an audit happen, but I have seen dozens of Soldiers receive the notification to provide missing documents.
What "warrants" this is a GAO requirement.
For you, and the centralized promotion boards, iPERMS is a record of your service.
For the GAO and Congress, it is a list of receipts, required by law, for every dime they spent on you. And that is the real reason why the law requires iPERMS to hold those records in perpetuity.