With Plusnet unique traffic systems there was a reason for knowing and using the figure.
It's because if we didn't our traffic prioritisation wouldn't work (i.e. the gold, titanium etc).
Traffic will come from the internet as fast as the given server can send it. This is usually much faster than the customer's connection can handle.
If we then slow that traffic down to the given profile speed, we can ensure that the information is sent on to the exchange and then the customer in the correct priority order. However if we allowed all the traffic through to the exchange, information would be sent in an arbitrary order meaning that we'd lose the ability to prioritise gaming or VoIP traffic over p2p for example.
Hope that helps explain things,