for me the primary motivator is to stay off of cell to avoid cell tower triangulation. the service I use right now actually doesn't even need a 'host' cell number. also, newer phones have multiple eSIM slots that you can rotate easily with the method I describe above, allowing you to further containerize your personas.

its different depending on what you're trying to accomplish. if you're trying to hide from the government, good luck. if you're trying to make it so Walmart and Amazon have no shared information on you, that's very achievable

It is difference. Sim cards can push arbitrary remote code execution to your device from towers.

That gap makes a huge difference