Some of the figures represent animals and it is easy enough to think why the Nazca Indians may have drawn them. They could have worshipped the animals much like other ancient civilizations. They could have just represented them, to the "gods" above, as animals that existed in the area. Some of the drawings are below.
This is a hummingbird
This is a spider (in the lower left)
A close up of the same spider
But at least one drawing has scientists a little baffled and then there is also the drawing of the astronaut....