There are three main listening styles. With a combative listening style, you're basically waiting for the other person to make a mistake so that you can attack it, or waiting for a break in the conversation to interject your point. With the passive listening style, you listen patiently to what the other person is saying, while feeling like you're more or less comprehending what is being said. More effective than either of these styles is the active listening style, in which you verify that you've not only heard, but understand, the message conveyed.
One of the most obvious reasons to master active listening is to assure that you have received the message as the speaker intended it. Not accurately understanding what is said can lead to mix-ups, misunderstandings, and damaged personal and professional relationships. When you apply an active listening technique -- for example, paraphrasing what the speaker has just said -- you get validation that you've accurately grasped the message as the speaker intended it.
People like to know that they are being heard and understood. By actively taking in both what they are saying, and also the emotions behind the message, you're creating an atmosphere of trust and mutual understanding. The speaker is more likely to be open and honest with you if he knows he's speaking to a sympathetic, active listener.
Active listening can be an invaluable skill in the workplace. Particularly during boring meetings which nonetheless convey key information, it can be easy to drift off into passive listening mode, missing much of what's being said. By finding an active way to take in the information, such as asking questions, the listener is more likely to retain it.
Rather than taking a combative stance in a disagreement, sometimes objectively mirroring back what a speaker is saying can help your debate opponent find holes in her own logic. Often just knowing that she's been heard is enough to coax her to soften her position and consider other options. If the topic is emotionally charged, it can be particularly cathartic for the speaker to know you've taken his message in, thus preventing "emotional flooding" or being overwhelmed by negative feelings, which can get in the way of a calm resolution.