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When baristas prepare espresso, they control how long the shot runs in order to achieve a certain extraction range. Ideally, this range is sufficiently long to pull out all the desirable flavors but stops short of bitterness and diluted mouthfeel. The only way to control this is by making the bed of ground coffee, or puck, of completely even density. Because water is lazy and follows the path of least resistance, it will find areas of lesser density and extract that coffee first, forming a channel.

In this picture, a bottomless portafilter affords a peek at a channeling shot. The lighter areas are in advanced stages of extraction while the reddish-brown areas are just beginning. There are still some dry patches where the espresso has not even penetrated the basket. The result is a shot that combines undesirable qualities from both sides of the ideal extraction range. Even if other variables are appropriately controlled, a puck of uneven density will cause inconsistent results. And after all, consistency is a barista's best friend. They have to trust that each shot follows the parameters that they set out. Otherwise, they would have to taste your shot before serving it to you, and no one wants to share a good espresso.