it seems you are asking two questions in one.
the first part, about configuration register boot field. this is used to tell the router 'how' to boot up. depending on the bits that are set in that field, the router will boot up with different options and with different features enabled or disabled.
please see the following link for more info on the use of the configuration register:
http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a008022493f.shtml
as far as ROMMON, it is used for a few different things. a few of the main things it's used for is a last resort boot environment and for router recovery purposes.
(for example, if your IOS is corrupted or cannot boot, the router could boot into ROMMON which would allow you to fix the problem or boot with a different IOS or even allow you to get a new IOS onto the router so it could boot; another reason to boot into ROMMON is for password recovery)
please see the following link for more info on ROMMON upgrade using the boot image:
http://www.cisco.com/en/US/products/hw/routers/ps214/products_tech_note09186a0080110ed1.shtml
please see the following link for more info on using ROMMON for password recovery:
http://www.cisco.com/en/US/products/hw/routers/ps259/products_tech_note09186a0080094a0b.shtml
