Why is 3D line graph used in excel?

Answer 1

With 2 dimensional data you are displaying two points of data on the X & Y Axis; however sometimes this can be limiting because sometimes there is a third dimension of data, so by using a 3D chart you can plot another series of data in the same chart relative to the other 2 dimensions.

This charts can sometimes be confusing but when it comes to certain types of modeling it can be very useful.