Network Performance, bandwidth utilisation, data flow speed, Bottlenecks, nodes issues, network medium issues, segmentation efficiency, distributed network requirement as a solution, multicast required as a solution or re design it.
It depends what your environment is. We have very good experiences with two solutions. When you're using Cisco Networking, their Stealthwatch solution (also part of their EA, a full NBA/ADS* solution) does a very good job and gets more and more integrated in their Networking, Security and Admission Control solutions. When you are looking for a less expensive solution, we have very good experiences with Flowmon, a spin off from the University of Brno (Tsjech Republic) and a very mature NBA/ADS* solution as well. In NL Flowmon is successfully in use by education, healthcare, finance and transportation.
*NBA/ADS: Network Behavior Analysis / Anomaly Detection System.