Wormly Metrics provides Network Traffic monitoring - among thousands of other metrics.
The amount of data traffic passing through your servers' network interface is a fundamentally useful thing to know.
Wormly reads data transfer counters and presents a graph showing you the average data throughput and its' trend over time:
Armed with this information, you can easily calculate your monthly bandwidth usage. Identifying spikes in data rates can help you spot activity (perhaps Denial of Service attacks) which might saturate your network connection.