Why the Figures and Lines?
picture from http://www.bobanddusty.com/nasca/
It might be easier to explain the figures that represent living things, rather than the geometric shapes or the lines. It is known that the Nazca Indians worshipped gods. it would make sense that they would make these "drawings" of such a size that the gods would be able to see them from the sky. As they were farmers, they would want to keep pleasing the gods so that they could continue to grow their crops. Although the area does not get much rainfall, there was plenty of water running underground from the mountains. They were able to make channels to get this water to their crops, and they did not want the gods to "shut-off" their water.