Just for info: in Spain it's compulsory by law for EVERYBODY 14 y.o. or older to carry a legal and valid form of ID at all times. Period.
"legal and valid form" can be either a national ID or a passport (or if you're an EU national, the driver's license is considered to be also a legal form of ID, but not so for US driving licenses, or US SS cards for that matter).
While it's highly unlikely you'll ever be asked to produce your ID for no reason, should you be involved in an incident (ranging from an accident, a brawl, or just being in the wrong place at the wrong time: a robbery, a terrorist scare, a police raid, etc) you might be asked to do just that and not carrying a form of ID can ruin your day because, in certain more serious cases, you'll be taken to the Police station until your ID can be checked out... or someone can produce a proper ID on your behalf... and a fine for not carrying your ID can also be issued.
But there are also practical reasons:
- Those youngsters wanting to go clubbing, even if your 18 or older, if you look like a spring-chicken you're going to be asked for ID to enter a club -well, they should ask you for it anyway.
- Proving your age to apply for discounts in museums or attractions for teens or for elderlies (a)
- Paying with your credit card. In some shops you must produce an ID alongside your credit card
- Renting stuff: ie a bicycle, a segway, a go-car...
- To access the services of a cybercafe (that's a new measure just approved to reduce potential terrorist threats)
Lastly, and this is just a personal reflection, it's simply a matter of self-protection: say your daughter goes on her own for a stroll and something happens (she's run over by a car for example). She'd be rushed to the E.R. by the Emergency Response Services of course and she'd be taken good care of, but without ID no relatives could be contacted if she's not conscious, could they? This is not to turn you paranoid of course, but being 'identifiable' at all times it's a good thing I believe, don't you? :))
(a) Certain attractions will accept other forms of ID too, such as a non-EU drivers license, or even a library card or university card providing there's a (recent!) picture of you and the d.o.b. is indicated.