Bo says it better than I would... from the readme:

"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."

Another must-have for scenario builders! 1.0.1 added the conversion of platform info (yes! you can now go back and forth between Forge and Pfhorte!) and a few other things. 1.0.2 fixes a bug which prevented the editing of some Pfhorte maps, and adds an "N" before network level names. 1.0.3 works around physics file weirdness. 1.0.5 added picture conversion from Win95 maps, and 1.0.6 fixes a bug in previous versions that eliminated notes. 1.0.7 adds support for tag sections.