Pressure Fermentation
Pressure fermentation is becoming more and more popular at the homebrew level, but in order to do it safely and accurately, you need a way to release excess pressure. You'll pressurize your vessel to your desired psi, but once fermentation begins and those hungry, active yeast start creating CO2, the pressure inside the vessel will increase. Attach a spunding valve and set it to your desired pressure, and it will blow off when the pressure gets to high and close up again when it's back to your desired psi.

Pressure Transfers
Even if you don't have a pressurizable fermenter, you may have done pressure transfers from keg to keg during filtering or after fining. During pressure transfers, it's a good idea to have a bit of head pressure in the receiving keg, especially if you're transferring beer that is already carbonated. Having a spunding valve in place will allow you to consistently control the head pressure and give you more consistent results, without losing carbonation in the process.

Diaphragm vs. Poppet
With the poppet style spunding valve, a spring pushes against a poppet to keep the valve closed. When the poppet lifts up to open, the surface area the gas pushes against becomes larger, and the spring has to work harder to push the poppet down and reseal the valve. This can lead to more gas being released than intended, and the remaining pressure in your vessel may be at a lower psi than your spunding valve’s set psi.