Backing Up and Restoring the XP Registry

There appears to be two levels of Registry Backup...one can backup (1) just the Registry Keys or (2) the entire Registry ( "System State "). I do both, but wonder why. It's my understanding that the keys are the most important backup, since the keys need to be recovered if you muck things up while modifying the registry. If that's true, then when wouldn't key recovery be sufficient such that I'd have to restore the entire registry instead (short of a total wipe-out where I'd have to reinstall the entire OS anyway)?

 

PeteC.........I guess my question isn't clear...I realize that SS is big collection of stuff, but when would I want to use SS and not just do a total reinstall? My assumption is that if I mess up while changing something in the registry, the keys are enough. I just can't envisage a situation where SS recovery is the solution (it's either more complicated than that or less complicated than that).