Just out of curiousity: what is the need for a NIE Certificate?

I have a NIE number and did use it sometimes (buy poroperty, open bank-account etc.) but nobody ever asked me for a Certificate. Most of the time i'm under the impression it doesn't bother wich number you give at all, as long as it looks like a NIE-number ;-). They love paperwork in spain so why not give them a random number when buying a new stove  ;D

Mark