Heart Rate is just another measure to use, along with distance, speed, elevation change, etc.
Let's say on day 1 you run 10 miles in 70 minutes with an average heart rate of 150 then on day 30 you run 10 miles in 70 minutes with an average heart rate of 140. Well then that means you have improved. Now, if you notice your average heart rate going up then you know something is out of order be it your training or the weather (ie: when it's hot as hell).
Use the same ideas when evaluating a track workout. Let's say you are running 10x1k in 3:15 with 400m 3:00 recovery.. You notice your heart rate going up to 180 during the fast work and down to 130 during the recovery. A good progression would be that you drop to 3:00 during the 1ks with the same 180 heart rate and/or you recover faster now getting down to 120 during the 400m recovery. Now you make the stress harder by either increasing the 1ks to 1200s or cut the rest to a faster jog (2:30 instead of 3:00)
Stress, adapt, increase stress, adapt, etc.
Alan