Why do you use box and whisker plots?

Answer 1

Because you can compare the values easily, for instance, you can compare the highest and lowest value and compare this to the mean, does the highest and lowest value differ greatly from the mean? Then you know the correlation is a bit unpredictable, you can also use this to compare two box plots, putting them together you can see through the median and quartile range the best way to do something, etc.