If your grease line maintains more than 800 PSI of pressure after greasing the coupler will have difficulty releasing. With a slight twist of the relief valve, your coupler will release a small amount of grease and allow the coupler to disconnect easily. Having the coupler and valve in one package is a cheaper easier solution!
See how it works here! https://www.youtube.com/watch?v=2LIiPYFCezw