I must start this off by saying I don’t know if this applies to all of Spain, but here in Melilla (North African Spanish enclave) they have brought in a new rule when applying for an NIE number. You now have to give in 2 copies of EVERY page in your passport when you apply, along with the other things from before (3 photos, application form & copies, original passport). I know the rules about applying for a number here are slightly different - an individual can’t apply for themself, it has to be through some organsiation eg. social security, trafico etc. mainly because they are dealing with mostly non-europeans here (Morroccans & African immigrants), so this may just be for here. It seems they want to see where you’ve been before and have a record of all your previous visas - big brother is watching you!