Listening is what to do when your scene partners are talking.

Listening is how you feed Your Personal Search Engine.

Listening is a willingness to change.

Without listening, you're not in the scene, you're not letting the audience see you react to what's really happening now.

Without listening, you might as well be doing stand-up. Without listening, synergy between the players is not possible.

Listening is what fuels the engine of improv. The more you listen to the on-stage action, the further you can go. Without listening, you're standing there dead in the water, disconnected from the scene.

Temptations

Even though Listening is where all the good stuff comes from, we're all tempted to do things that block us from listening:

Thinking of your next line (while your scene partner is talking).
Checking to see if the audience is laughing.
Searching your mind for material.

A great Exercise to practice Listening is Cocktail Party.