A phone number, at least in the UK, means you've been pre-verified in some way - users can't in general generate new phone numbers like they can email addresses.

Thus, less problems with anonymous users (eg trolling, spamming) and less abuse from named users as they can usually be traced using the phone number.

This seemed interesting to me, so I tried signing up for a UK voip number at the sipgate.co.uk site. They do ask for an address, but they accept anything valid, like the address of a university. Had a 056-0003 XXXX phone number in less than a minute.

Most countries are moving towards requiring identification before activating a SIM card, like you say.

For example in Vietnam, this has only become a thing in the past month or so and it is not enforced in practice in many unaffiliated corner stores. Last month, I just walked into a corner store paid a couple of bucks to get a preactivated SIM card with a couple of GB in two minutes.