MCC_sandbox allows one person (anyone who has the mod installed) to become a Mission Maker (or Dungeon Master) allowing them to do basically anything to the mission you are currently playing (eg: add enemies, add supply drops, call artillery, request pickup, call air support, spawn cars, change the weather, spawn suicide bombers, civilians, etc).
Its quite good as long as one person is designated the DM who is in charge of running the mission making it more interesting for the others.
However it can be a problem if people just start playing with the controls while you are running a serious mission (eg they could just spawn ammo for themselves if they are running low).