Why does ford use coolant in the throttle body of 1998 ford ranger 2.5l fuel injection engine ?

They ran coolant lines to the throttle bodies for emission purposes the coolant actually is used to heat the air entering the intake manifold to help preheat your air fuel mixture so less hydrocarbons are made during the combustion cycle however this preheating reduces actual horsepower produced at the flywheel by as much as 10% according to many dyno tests, there really is no proof of any mileage gain or other benefit to this design add , on the older cars and trucks if you bypassed this you had to readjust your timing so you motor would not ping or get a detonation knock under acceleration

Are you asking why coolant and not something else? Or why they cool the throttle body?

Well, there are a couple of reasons First off, look at what is going on under the hood. The way things are configured from the factory radiator fluid runs through the throttle body to warm the incoming air. Supposedly this is to keep the throttle body from icing up in cold conditions.