This might be a strange question considering the original inspiration was a dream I had, but here goes.

An alien race suddenly appears and immediately declare that their intention is to conquer Earth. However, they are surprisingly amicable about it - they arrive, make known their intentions, maybe meet with high ranking politicians from various countries. They give us time to prepare, but either imply or outright show that we cannot fight them - their technology is too superior to our own.

My question is - why would be a probable reason why an alien race would want to begin (and presumably end) diplomatic relations like this? My own internal answer at this point is that the aliens are in an "Alpha Island" society, similar to *Brave New World*, where everyone is high class and they need us to serve as the lower class. However, I feel like this may be a weak justification for the strange proceedings.

The scope of this question mainly deals with the why - other factors such as Earth's response (futile war a la *Independence Day* probably) and other social factors may be asked in later questions. For this question, you may assume

 - The aliens arrive in the near future, with maybe a hundred or so ships over populated cities.
 - They have perfect knowledge of local languages, so communication is not a problem.
 - Along with the previous, aliens try to meet with the everyman - they may even go door to door in suburban neighborhoods. 
 - My current idea is for Earth to be human-less once the aliens leave, but this point is undecided.
 - Alien weaponry has not been demonstrated yet, but has immense destructive capabilities - a war would be ended quickly.
 - The aliens would prefer to avoid bloodshed, but will cause as much as necessary until their goals are realized.

So, what goals might these extremely polite conquerors have?