Traffic Avoidance Settings

I have a new Garmin 2820. I understand that the traffic avoidance feature is only as good as the city's update of the traffic information. What I don't understand is the settings on the avoidance option screens. I called Garmin, since the owner's manual has no reference to it's use. Garmin tells me that the settings are used to tell the user how far in advance the unit will warn you in advance of traffic. Well, that doesn't really make sense since the settings start at about .2 miles and go up to several hundred miles. Why would I care about traffic avoidance 200 miles down the road? In the "auto" setting, what is the traffic avoidance looking at? Why wouldn't I want to set it at 5 miles, or 10 miles or ???. Anyone have any ideas?

Thanks a lot. Great web site.