A distribution can't be described fully only by a measure of central tendency. An index of dispersion is also needed.
Variability is a description of the tails of our curve - that is of the spread of scores.
25 Mean is the same on both, but very different distributions. If just H - L it would be 3, so add 1. So range can be helpful, but is the most unstable measure of variability there is. Because only affected by 2 scores. There are other types of ranges used.