It's similar to changing socks while running. One cannot replace fundamental components of the operating system while they are in use. Phase 1 can build an image of fundamentals that can then be started in a controlled sequence. Phase 1 also builds a checkpoint. And, attempting to keep user programs running while the operating system is being shuffled out from under is very difficult. For example, early in the boot cycle the full file system is not yet running and this is a good time to update that component.