Why does the throttle body on my '90 Ford LTD Crown Vic need coolant connections

Your car was made a third of a century ago, and strange things were done then. When throttle bodies have coolant lines it's to prevent the effects of cold weather on them; they might otherwise freeze. If you drive in warmer conditions where you don't see bitterly cold winters, you can plug them. The original idea was that a choke or choking operation managed by the computer would effectively work the intake until the engine warmed up. Thereafter, cold air might impact the intake.

It’s main purpose was to prevent icing up in the throttle body and upper intake. Where I lived at one time this was a major problem especially in late fall when the early morning temperature was right at 34 degrees and high humidity.

Exactly, in the old days a phenomenon known as "carburetor icing" could occur in cool, damp weather. To combat this hot exhaust gasses were directed through the intake manifold, under the carburetor, to keep the throttle plates warm until the engine warmed up. Later on hot coolant flowed around the throttle body of fuel injected engines, like yours, to prevent the throttle plates from getting ice on them. This was used for many years. My 2000 Grand Prix has coolant passages going around the base of the throttle body as well. Hope that helps! Jim