It's only used on about 4 maps in the entire game. I wanted to create heavy contrasts in Miehlebin- the strange, creepy town, the bright, sunlit forests of the lower part of the mountains, and the dark, foreboding paths that lead up to the Reactor.

So pretty much Town=Unfamiliarity, Lower Mountains=Reminiscence(works better with the music) and Upper Mountain=Foreboding.

Looks better with the proper screentone anyway.