keeping a lower heart rate ( or the lower end of your range), helps burn fat as the main source of fuel. your body doesn't really like to use the fat as a fuel, and prefers using glycogen, but if you want to burn the fat, this is the way to do it.

i also know that working at the lower end for a longer period is mainly for endurance training. the idea is that if you maintain a steady"lower" heart rate for a longer period, you can go further and further.