SMOG PUMP ON 190E 2.6

I wanted to replace the malfunctioning regulator on the back of my alternator. On my car, facing the radiator under the hood at 2 oclock the water pump ..at 4 oclock a/c compressor...at 9 oclock the alternator and at 10:30 oclock what I thought was an air pump is a SMOG pump? WHY ? and how is this part necessary. This part is not even listed in my MB parts book, micro-fiche, Haynes, or Chilton manuals. I found the the part in ARCHIVES Q/A and it costs 700.oo if it should go bad Does any one else have this item , or know how it functions? (same serpentine belt) engine is M103, year 1992, thanks for any assistance or info ............Rick (a little larger than alternator with hoses)