The M5 East tunnel in Sydney is a great example of why that information would be useful for a navigation system.
The tunnel was completed in 2001 with enough capacity, as stated, to last 20 years. It started to exceed its capacity within 5–10 years of completion.
The position my Garmin is usually fairly accurate. This is probably because the slowing of traffic begins before the tunnel while the device still has signal. When the traffic is at its worst, however, the Garmin decides that since I was moving at 25km/h when I entered the tunnel, I should have been out of the tunnel by now and thus there must be something else blocking the GPS signal—when in reality I'm still stopped somewhere in the tunnel waiting for the traffic to move again.
If the stated information was available to the navigation calculations, it would know that I had slowed, stopped, moved a bit, stopped, inched forward a bit more, stopped, stopped, still stopped, grown incredibly impatient and started to curse the government which built a tunnel with very a short life expectancy and without any thought for expansion, etc.