First reason
A glow engine does suck the fuel only ¼ cycle time
But the other ¾ cycle time, while the engine is compressing, burning and exhausting the mixture, what is that pump doing on that moment? Waiting? No want the spray bar isn’t closed and the piston is pulsating a second time the crankcase pressure.
I think that the standard pump, as we know now, with it's constant (about) 0,25 PSI pressure, is pumping a second moment a little bit fuel in the engine near the closed inlet valve.
And that fuel maybe is waiting to blow out that little tiny candle in the cylinder, we all call the glow plug, the first moment the valve opens.
When people use the regulated pressure pump on a four stroke, there are another two things to consider that often makes this work.
1. You must tee the line from the crankcase to pump with a drain line to allow oil to get out. This weakens the pulse, but this is made up because it gets to pump twice.
2. You must bypass the line from the carb back to the tank. This reduces pressure so that the carb will only take fuel up from its on suction.