Because many departments have pancake adapter/ reducers on their 2.5” discharge and have an 1.5” discharge somewhere in their station. So all you need to do it connect a 1.5” or 1.75” hose to that and open up take fill. Instead of finding a reducer from 4-5” to 1.5”. As for the second part of the question, I agree, why would you want to?
On trucks our department had many years ago (1976 Pierce & 1968 Seagrave), this would be done in the station after a call to fill the tank.
Yes it can be done but whyyyyyyyhy
Because many departments have pancake adapter/ reducers on their 2.5” discharge and have an 1.5” discharge somewhere in their station. So all you need to do it connect a
1.5” or 1.75” hose to that and open up take fill. Instead of finding a reducer from 4-5” to 1.5”. As for the second part of the question, I agree, why would you want to?