I have just purchased a used beseler mcrx with a dichro dg head. everything appears clean and in good working order. There is a part to it though that is confusing to me. There is this separate piece that is a box that is to be connected to the actual enlarger head by a hose. Being new to all this (having my own equipment...) I am wondering what is the purpose of this unit. Is it a necessity, etc.

Is it a cooling fan? If it is you'll need it too keep the thing from overheating.

Yes. It is made to be mounted separately, on the wall, and connected via a hose so that it will isolate any fan vibration from the enlarger.

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It is the cooling system for the head, and needs to be put in the loop. It goes on when the timer is activated.

Actually I have mine set up so the fan blows continuously when the enlarger is powered up, only the lamp is on the timer switch. That reduces the load on the timer circuit and cools the bulbs (there are two 200 watt bulbs) a bit better by keeping the air cooling after the timer cycle is done. Same reasons as you would for a slide projector. You should also have a power box that is a separate unit, made to ride on the enlarger chassis. I think it is just a transformer and not a regulator but I'm not expert on this point.