What do you do if your users complain about the poor response time of a core application?
What are the bandwidths of your WAN and internet lines? Is there any potential for savings?
When do you need to upgrade your data center to the next ethernet generation?
You can only answer these and many other questions about performance if you have full visibility in your network. You need to know who transmits how much data when and via what protocols and channels, and what this looked like in the past.
However, measuring the bandwidth on all WAN lines is not enough in this regard. Only more highly developed tools offer you the capabilities you need to keep track of performance at all times.