Most network monitoring software can retrieve lots of data about a device using SNMP. In particular, it's quite easy to get the volume of traffic flowing through various interfaces by reading the statistics from MIB-II. This shows total bits/bytes per second flowing on that port. This is extremely valuable for looking for overloaded links and for other kinds of trouble.
However, these SNMP stats won't answer the big questions, "How is the link being used?" or "Who's hogging the bandwidth?" For that, you need a more detailed look at the actual sessions, which is exactly what NetFlow is designed to do.
