 What seems to be the invader's reason for visiting Earth?
For the Invaders have spared no part of this planet in their depredations: they have laid Hong Kong waste; they have terrorized London; they have destroyed the lives of citizens in every member state and in every inhabited area. "Since the Invaders first appeared in Wood River, Oregon, 24 hours ago, they have not once acknowledged our attempts to communicate, or in any way taken notice of our existence as reasoning beings. They have examined us, dissected us, driven us mad, killed us with no discernable provocation; and this is more intolerable than any normal invasion. Not only our individual lives are threatened, but our existence as a race. The Invaders were floating up the sloping side of a tall, pure white structure that dominated the rest.
 They are investigating humans in a scientific, albeit fatal, way.
 They recognize humans as intelligent beings and wanted to see what they have made.  They want to wage war with Earth and take it for themselves.  They are investigating humans in a scientific, albeit fatal, way.  They are investigating humans, making notes to not destroy their world indiscriminately.