The MSA Gallet and the MICH are the same thing, the TC2000 made by MSA. The ACH is the big Army designation for the MICH. The ACH is also produced by SDS, and a very similar if not identical helmet is produced by Gentex.
As the military MICH name implies and others have mentioned, the helmets purpose is too allow the use of specialized comm gear. It is also lighter and has a better suspension system than the PAGST. Fragmentation protection is the same as the PAGST, protection from small arms is slightly increased (NIJ IIIA), and the improved suspension system offers better protection from bumps. But, the fact that fitting it properly to ones skull takes extra time and care, and the decrease in coverage from the PAGST, rates of head injuries in the Army are up since it's adoption.
The Marine Corps has wisely chosen to go with a lightweight version of the PAGST for the extra head and neck coverage it offers with the exception of some units which need the comm capability.