It’s much much cheaper to conserve bandwidth than it is to buy more. The goal of usage monitoring is to help you identify the kinds of traffic on your network so that when delivery monitoring tells you that you have low available capacity, or experience monitoring tells you that application performance is poor, you know whether high network usage is the culprit. For example, usage monitoring can help you discover that traffic volume for ‘streaming media’ is high; that it’s consuming X Mbps of bandwidth at 1:30pm every day, the same time that other performance indicators are tanking; that 192.168.0.10 is generating that traffic, and—if you’re set up for it—who was logged onto that machine at the time.