"what is the electromagnetic clutch used for? My tractor does not have one. Do I need or want one? "
You are posting in Power King, IF you have one of those, it probably is equipped with a good manual PTO engage which is better anyway since they can be engaged slowly, smoothly, while and electric PTO is "bang" fully engaged.
Just to further comment, I'm also sold on manual vs. electric. (If given a choice)
The John Deere 400's had a manual PTO, and you know with that company and that era with shaft driven decks and power steering, if they had felt that an electric PTO was a better choice they would have went with it. Case 446 types also had manual PTO's
The two later Powerkings I had both had electric PTO clutches on the front of the engine. I imagine yours has nothing at all if it doesn't have an electric PTO clutch?
I'm not familiar with the pre electric clutch Economys.
1973 was a big transition year for Economy. Some 73s had the old sheet metal with manual PTO clutch. Some had the newer sheet metal with manual PTO such as the one in question. Just a series of pulleys sticking out the front of the engine. Some 73s (like mine) have the new sheet metal and electric PTO clutch. It was a funny year because you can literally have one Economy that looks like an older style, actually be newer than the one with the new sheet metal sitting next to it. They used the parts as they found them until the old stuff was gone.