Ezechiel
Paranormal Adept
1) Scholarly knowledge or curiosity
2) Military threat assessment and planning
Honestly, every scenario I can imagine fits into one of those 2 categories. Number 1 can be nice and friendly or cold and calculating. Number 2 is chilling.
From a human perspective (given our level of advancement) these are scenarios that make sense. The other one missing is covert exploitation. (As we evolve it becomes harder for them to keep the cover lol).
These scenarios would be applicable to competing civilizations relatively close in capabilities to earths.
IMHO, the level of advancement of a civilization dictates needs and requirements that sustain a civilization. These needs translate into intents and actions that solve their problem.
For example: Chinese presence in Sudan, American presence in the middle east are required to secure energetic resources that feed their respective empires. Along that line of thought there could be a multitude of failed eco-systems in the galaxy where some civilizations are forced to rebuild using compatible material taken from remote and viable planetary systems (Speilberg's ET could fall into that line of thought)
This would depend on how fast complex organic structures can degrade... Is there such a thing as a genetic weapon ?
http://news.bbc.co.uk/2/hi/health/259222.stm