Theres a vast amount of information that cant be covered here but the main thing with the "why, what for question" is that ancient civilizations particularly the Sumerians, Mayana, Dogon tells us that they got all their information from the "annunaki" which is the sky gods. Theres always going to be a question you can ask that pro-alien people cant answer. The thing is theres all sort of ancient civilizations that were all isolated at the time that all have similiar legends, monuments or writings . It just cant be a coincidence.
For me the single best proof that aliens visited or at least exist are the dogon people.
http://www.ufoevidence.org/topics/dogon.htm (you need to look up other stuff on the dogon its not all in this link) Quick Summary of them. The Dogon claim that they were visited by aliens thousands of years ago. They have extremely astronomical knowledge without any type of technology. Claim they got the knowledge from the alien visitors. They told a western researcher the location of a star named Sirius in the goldilocks zone. Around 20 years later Nasa verified this claim by identifying this star with a telescope. Now heres the crazy part, this star is not visible with advanced telescopes. So how did they know the location of this star for centuries? The Skeptic response is that europeans taught them this astronomical knowledge which is simply nonsense.