Need for more flexibility
Sometimes basic stem and leaf plots are not flexible enough — there would be either too many or too few rows of leaves to show the varying density well.
Repeating each stem 2 times (with leaves 0-4 on the lower copy and leaves 5-9 on the upper one) or 5 times (with leaves 0-1, 2-3, 4-5, 6-7 and 8-9 on the different copies) gives intermediate numbers of stems.