If and when they make their presence known, it's likely because they want the Earth's natural resources. If they've already been here covertly, they're likely to only be here for observation (if they're scientists) or reconnaissance (for future attacks or ensuring we're no threat yet). That said, getting here would be a huge feat in the first place which means their tech would be far beyond ours (and they'll be much smarter) and they're determined enough, which probably means they want precious resources or they're very concerned about us.
Although sailing over the Atlantic is much easier, looking at what the Europeans did to the Americans during the colonial days, the exploring and travelling between was still a feat that was driven more than just discovery. So one can imagine that all but the most advanced and well-travelled beings won't be just dropping by to say "hi". Whether it's gathering resources or information, there's bound to be a bigger reason and probably not in our favour. The question is, are humans just particularly cruel and greedy, or is it an inevitable trait within interloping/expansive beings?
If they're really advanced, and staying out of sight, then it may be in both of our interests to maintain the status quo which is why they're doing it. The less advanced they are, the more they'll need to take stuff from other planets and the harder it is to get there, thus the more likely they'll be here as a threat. So let's just hope they're the former and we'll not hear from them anytime soon...or not?