Well you only need those startup disks if for some reason you lost your bootable disk but you have the windows setup files. Sometimes, vendors will give you a copy of windows xp on a disk but it won't be bootable. Or sometimes your having a problem booting to the cd (maybe the cd is scratched). But your right, usually you would not use the startup disks unless you absolutely had to.

If you can't boot your system, you should just use your bootable cd and start setup that way. You can then do a repair installation from setup and it should fix your problem.

The boot disks are a last resort if for some reason, you just can't get your system to boot to the Windows CD.