As an adult, you probably need to authenticate yourself in the following situation.

opening a bank account, getting a credit card, getting a mortgage or a loan, buying a flight ticket, signing up for internet service, signing for mobile services, buying a concert ticket and the list goes on.

What's common here is the service provider need to know you are actually the person say it is you and not someone else.

Back in the old days where we apply the service in person, you can take your driving license or passport to authenticate yourself, but with myriad of services now moving online, we need a centralised system that mimic the physical ID.