Her reasons were unstated in the drama, although personally, I don't think she came to Earth to invade it. This is probably a topic that would've been touched upon if Circle had more episodes than 12. The writers decided to focus on the subject of "memories" and create a cliffhanger for the whole alien plot to acknowledge its existence for us the audience, but at the same time, say that don't really have enough time to create a subplot. I'm pretty sure this would have been a subject for a non-existent Season 2. :D