Mean poor because pulled away from bulk of scores by extreme scores. Mode is poor because no info on anything but most freq. Mdn. is best single measure because it is between mode and mean. 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.