The default way for people to prove their identity
For most government services, they really only need to know who you are if they're giving you money. - for that, these days, they require you to have a bank account.- or you're giving them money. Either way, the bank/payment processor have already done an identity check. For anything else (like checking your tax bill or NI contributions) they would historically simply have written to you, so why they suddenly need documentation they have never previously required is beyond me. Maybe we should just take the Norwegian approach and make everyone's tax returns public.