Actually, what Herman Wagter of Amsterdam's Citynet said at F2C 2009 was: latency is the cause, bandwidth is the effect. But his explanation matched my title above.
If you are attempting to interact with other people, whether by VoIP or just playing cards together (with video) you need less than 200 milliseconds of end-to-end delay. If it's playing cards together, with video, and you need to exchange 500 Kbps in less than 200 ms, you need a 100 Mbps pipe!
It's latency that drives the need for high bandwidth. Most people won't fill that pipe most of the time, but they need the pipe to guarentee that what they do send gets through rapidly.