-nomap is simply a different method that the game engine uses to map the addons into the memory.
When OFP was first coded, the hardware available was of much much lower power and capability, thus the game code was not written to cope with large numbers or sizes of addons. The -nomap command uses a more "modern" method of mapping the addons into the memory, which allows the use of more powerful hardware like the latest GeForce or ATI cards.