Click-to-dial allows a user to click on a web page to speak to a customer service representative.
The web server then creates a call between the user and a customer service representative.
Similarly, the usage of early media (where session data is exchanged before the call is accepted) with third party call control is not trivial; both of them specify the way in which user agents generate and respond to SDP, and it is not clear how to do both at the same time.
It allows for re-connection of the media stream to an alternate end point if the original destination end point becomes unavailable without dropping the original emergency call (which has a separate media stream to the 3PCC server).
Additionally, it minimizes bandwidth requirements between the 3PCC server and the emergency call handling end point because all mixed media uses the same RTP stream.