What's the reason why you have to have a SIM card to activate an iPhone?

What if I want to use my phone after my plan gets canceled. Is that legal without a sim? And is it legal to use someone's sim to activate it if my phone software brakes. But what's the reason that you need one to get into the iPhone? That doesn't seem right at all to not allow someone to get into an iPhone that has one.

It enables the carriers to activate their carrier lock if the phone is supposed to be locked to their network, sadly this is the case. I had to wait ages the other week for my phone company to connect Apple to remove the sim card lock, which is interesting.

Once it's activated, that's it - it is a one-off thing unless you do a "factory reset" on the phone, eg. In preparation for selling it.

It will not stop working if you take the SIM out or just leave it in after the plan finishes - you just won't be able to make phone calls.

You can always get a cheap pre-pay SIM and not put any extra money on it, if you need to re-activate the phone after doing a full reset. (The cheapest in the UK are 99p - around $1.50).

Don't forget to ask your existing service provider to unlock it, before the contract ends.
The phone is far more useful and worth rather more without a network lock.

Because it is a phone after all & so the usual intent is to use one as such.