Title says it all. My favorite use was to use it to extract the Reaver physics from M: R and go all Godzilla in the net levels. >:)
This is a handy little OS 9 program that will un-merge a map file.
From the read-me:
Marathon Map Splitter takes a merged Marathon scenario file as produced by Forge and splits it into individual map, physics, and terminal text files for each level. It also extracts any resources into a "Terminal Picts" file. It is the exact opposite of the "Merge Levels" command in Forge. This lets you do things like peeking at other people's physics models (ever wondered how to make a potatoanus?), fixing the bugs in Blood Tides of Lh'owon, and converting old Pfhorte-generated scenarios.