melonfairy As someone new to this space, what exactly is the concern here and what would be the threat model where you would worry about such things?
If someone who knows your real identity is interested in tracking down exactly where you are, you might want to make sure neither your IMEI nor SIM is linked to your true identity (credit card number, social security number, real name). The cellular network work such that one can always locate exactly where a device with a certain IMEI or SIM is, down to within 100 meters from the actual location.
Domestic abuse victims who have fled from their abuser is one common example of people who may want to be able to use a cell phone without their abuser being able to track them down. But I can imagine some activists in repressive regimes, that are living hidden because they are already known by and sought after by the authority, may also have a need for decoupling their identity like this. Same for their families. Using no cellular connectivity and only public Wifi access points may not be feasible for these groups, as they are just trying to live a normal life.