What is the reason for two passport photos?

Passports only have one photo on them, however when you apply for a passport, two photos are required (authenticated by a responsible person to be a true likeness). One is used on the passport and the other is held in the records of the passport issuing authority (so that it can be checked in the future) along with the central records of the issue of the passport.
In future it may be possible to check this central picture against the one in the passport as you enter a country so as to prevent stolen passports (with false pictures in them) being used.