I suspect it's there because lots of heart rate monitors track it, and it's a pretty good gauge of how hard a particular ride was. I track both Average and Max, and use them mostly to ensure I'm going slow enough on "recovery" days. I have a tendency to go out hard all the time, so ensuring that my Average Heart Rate is near my goal for the ride often helps me keep from going too hard when I shouldn't.
It's also useful for gauging your efforts on longer, harder rides. If you know what you've averaged on similar rides, you'll have an idea as to whether or not you should push the pace or slow it down a bit in order to survive.