E.T

I saw the movie "E.T" for the first time.It was a kind of heart-warming movie between human boys and a NOT human creature. Human boys helped E.T to get back its own planet safely.If they had not found it,it would have been caught by adults who wanted to study it,and it could not have gotten back to its own planet.
I thought E.T was cute.Especially,the way of walk was impressive.Its feet were very short and when it walked,it needed to shake its whole body to reach out its feet.When it mistakenly drank,it bumped against a desk.This was cute,too.
I found how the moon was shown in the movie important.From the scene that a boy found a boll come back ,to the scene he saw E.T for the first time,the moon was very round and never move.It was impressive.And when boys flied by E.T's power,the moon played an impressive role,too.
I have a question about this movie.Why E.T came to this planet? What was their purpose?