Whats that thing?

While changing and idle control valve, i noticed that on the left side of the motor, on the bottom by the crankcase, is a thingy, looks like some sort of pump or motor that is supposed to be driven by a belt from the crankshaft pulley, but the belt is missing, and looks like it has been missing for a while by the looks of the rust on the pulleys. It has a couple hoses coming of it, primarily one hose that goes to the intake manifold. Can anyone tell me what this is? What affect does it have on the motor or intake air if any? If it does have an affect and seems to be an important piece of the motor, i would like to throw on a belt for it. It looks to be the same size as a starter. If anyone can tell me what this might be, i would really appreciate it.

1985 500 SEC