Without a blowoff valve, when you close the throttle, the compressed air has nowhere to go but back through the compressor. This causes compressor surge, which is when the compressor wheel stalls out and makes a neat warbling sound. With a blowoff valve, when the throttle is closed, the increased intake pressure is released through the valve instead of being pushed back through the turbo.

...on a gasser.

On a diesel motor, there is no throttle plate to close. The accelerator pedal modulates fuel, not air. When you let off the pedal, all that compressed air still gets pushed into the combustion chamber. This means that there is no air charge reversion, thus no compressor surge, thus no need for a blowoff valve.