What is the purpose of SSL/TLS renegotiation?

Question

What is the purpose of SSL/TLS renegotiation and under exactly what circumstances does a renegotiation occur?

Please note I am not asking about the vulnerabilities associated with a renegotiation, there are many sources that are talking about this.

Answer 1

It occurs after either side has expired the session and continues sending data. It means either that the session has simply expired due to timeout, or that a peer wants to change the cipher suite, or wants to request a peer certificate and hasn't already done so.