I get the front part, but why is the back curved too? I do not see a problem with the back being flat.
The reason why the aft ends of airplanes are streamlined is to preserve a smooth flow of air. Just as the fore ends of airplanes are streamlined to smoothly cleave the air, so too the aft ends are streamlined to smoothly reintegrate the flows. Turbulence is bad, regardless of where on the aircraft it occurs.
Note that there is more turbulence behind the stumpy shape than the longer one, despite the fact that the front ends are shaped the same. That is why the back ends of aircraft are streamlined.