The IPS run a passport checking service, so people can phone up and verify that a passport number is valid and hasn't been lost or stolen. Perhaps that's why they want it? A similar service doesn't exist for other forms of ID (which is another reason to avoid using a passport as ID, because of the likelihood that IPS keep records of these checks).
http://www.ips.gov.uk/cps/rde/xchg/ips_live/hs.xsl/563.htm