The trap stops flue gases from entering the drain pipe, and allows free flow, or stopping "water slugging". That's also why you have to trap a/c condensate with the evap coil on the negative side of the blower.
The trap is on the negative side of the blower. if the trap was not in place, the blower would suck air up through the drain line not allowing the condensate to flow out. thus causing a flooding problem.
Just following what my HVAC man told me. Makes sense.
I hope this helps.
Sounds as though many ideas. Mine works and have had no issues for 30 years.