The fuel bowl is there to separate water from the fuel. Once separated, the only way to drain the water from the bowl is to use the drain valve. If you plug the drain valve, you'd have to open the top of the bowl and sop up all water and diesel with a rag until the bowl is empty and dry.

If you get a tank of contaminated fuel (happens more frequently than you might imagine) you'd be stopping quite a bit to empty the bowl until you could get home and have the tank drained. If you're on a trip or vacation, it'd be even a bigger pain.

Stewart