makes alot of polygones 1 polygone, wich will speed up, and is better to texture, mostly used when working with 2d shapes, cause then you get all these bsp cuts in the brush,dunno what they are called in a 2d shape.
though, you can only merge polys if
-they are on the same line, so no floor and wall togheter, but differnt pieces of wall
-if they all are part of 1 brush(not sure about that)
-if the polys have the same texturealignments, and thje same texture, and other things like translucent or not, ..... they all have to be the same of they wont be merged
-....