IdrisSeabright wrote:
If you don't transfer the SIM, you'll need to change the SIM number in your account by contacting your carrier.
I believe this is why you prefer transferring your SIM card.
In my case, I’ve always been with AT&T. When I order a replacement iPhone from Apple online, I am asked which carrier and answer “AT&T”. I can then submit my telephone number and some personal identifying information, and, lo and behold, when the new iPhone arrives, it is preset to expect to be placed in service using my telephone number, and the service switches to the new iPhone easily and quickly as part of the setup process.