Walbro Fuel Injection pumps are submerged in the fuel tank of tune port injected vehicles. The fuel injection pump requires a high pressure-high volume fuel pump and has a return line, this way all of the unused fuel returns to the fuel tank rather than staying in the line. This combination causes everything to happen at once-if fuel washes away from the pick up point even for a short time a block of air gets to the motor almost immediately, causing the motor to stumble or die. It's also hard on the pump because the fuel is the lube for the pump.
That is why an in-tank fuel pump is recommended. The in-tank pump has a trap built around the pump to retain fuel at the pick up point. The submerged pump also acts as a heat shield from the road and exhaust heat, which can reduce the chance of vapor lock. This combination give you a neater installation and appearance for your vehicle.