From
wikibooks-dirty
Enables ambient occlusion or "dirtmapping".
Basically, less-visible areas (such as nooks and crannies, cracks and crevices) will become darker.
This simulates a dirty, grimy effect.
and also there is
-dark
Enables darkening of lightmaps at brush/lightmap seams.
Sort of like a half-assed occlusion pass, ends up looking a bit Unreal 1-ish. Very subtle.
I used sometimes -dirty but i prefer no dirty, but sure is a personal preference