ORIGINAL: gaRCfield

Most engines get their fuel from vacuum pressure formed in the cylinder during a stroke. This is effective when a fuel source is near by, however the suction is not great enough to draw fuel through fuel tubing from a distance. The further the fuel tank is from the engine, the more force is required to push fuel through the lines.

Fuel tanks in RC planes are usually placed behind the firewall for this reason. In some applications it is not practical or not desired. Twin engine planes sometimes share a central fuel tank, sometimes there is no room behind the firewall due to airframe design, or sometimes people prefer a plane with a constant center of gravity - one that does not change as fuel is used.

Pumps allow the tank to be placed a greater distance from the engine, or maybe even below the engine. A pumped engine can still be used if the tank is near by, so it is just more versatile than a non-pumped engine.

Another advantage is that pumped engines don't really lean out in the air. They can be tuned to max RPM, and maybe just a few RPM rich to be on the safe side. There is no need to tune down 300-500 RPM with a pumped engine like you would on a non-pumped engine.

I would say the only disadvantage, if you want to call it that, is having more parts. All engines need to be primed, it just may take a few extra flips on a pumped engine to get fuel throughout the whole system.