I love how researchers try to understand why they were created: astrology, navigation, etc. Now we have this cat. Why do people post pictures of cats in the internet? No other reason than that people like cats.
I bet they just liked cats too.
Yeah this has always been my take on it as well. From what I remember the earliest lines are just lines and geometric patterns usually seemingly to do with water sources and navigation. Once they mastered the technique for long lasting pictures in the terrain why not start making fun things like cats, condors, and spiders.