A blow off valve is indeed used to release the pressure built by a turbocharger under boost. The reason for this is because the turbine and compressor blades of the turbocharger spin at an ultra-high speed (80,000 - 100,000+ RPM) and as a result contain a significant amount of kinetic energy. When you release your foot from the accelerator, the throttle plate slams closed, effectively cutting off the air passage to your engine. The turbos are still spinning however, due to their inertia, and are suddenly trying to push air past a closed throttle plate. Without a way to bleed off this excess pressure, the turbo blades would undergo shock loading and considerable damage may occur from this happening at such a high shaft speed. The blow off valve is used to vent off the pressure in the intake in these cases; this extra air can be routed to the open air in the engine compartment, which is why you hear the loud "pssshhhh" from some turbocharged cars.