With coffee, while making the shot, when the gicleur provides a sort of preinfusion. The valve slows down the rate at which the empty spaces above the puck and in the group fill with water at the beginning of a shot. During this fill up time, water trickles into the puck without full pump pressure. The low pressure water tends to seal any cracks in the puck and thereby reduces the chance of channeling.

Oddly enough, in the 80s the Saeco machines didn't have a restrictor PF, but instead a PF on which the flow could be turned on and off. By shutting off the flow at the start of the shot, while the pump was on, one could soak the puck for 6 to 7 seconds and get quite good shots of honest espresso even with dodgy grinders and poor skills. I'm not sure why they ever went away from that system