@Curtis W. Rendon "...why I would ever bother to buy a SIM card"
To change network if you decide that the tariff of your current network operator is no longer the best for you. You might also want to be able to change to a local operator's network when abroad instead of paying roaming charges.
If there is a return to the association of the phone's electronic serial number to a given network (like in the days of analogue cellphones), do you think your operator will help you change networks at will?