According to RFC7329 there is a need for having a globally unique session identifier for the same SIP session that can be consistently maintained across SIP Proxies. There is a Call-ID header value already as a globally unique identifier but in practice it is often changed by SIP Back-to-Back User Agents. Therefore in order to provide an identifier that will not be modified/replaced by B2BUAs a new SIP header "Session-ID" is added.
The general concept of "Session-ID" header is that the UAC generating an out-of-dialog request generates a new, unique value that remains constant for the duration of the transaction, any dialog created from that request or for a registration.