Aliens are looking for fish in the vastness of space, using their signals as bait. Human beings are the fish.
Once we answer back with the correct signal, they will know we exists and eventually find us. It will be the end for us.
Stephen Hawking warns that humanity should not respond to aliens in case they kill us all
My response to that would be: aliens would have no harmful interest in us.
- If they wanted resources, planets and asteroids with no atmosphere would be much easier to mine than Earth. Also, Earth's humans would be like trying to pick fruit in a rain forest- mosquitos. Why bother?
- If they're looking for a habitable planet, it's unlikely Earth's atmosphere would be compatible. It took life billions of years to adjust to the amount of gasses in the air. The chances an alien species breathes the same concentration of gasses is very low.
- Having alien DNA would most likely make anything on the planet incompatible as food. If you think about it, we can't eat most of the other life around us...what are the chances of an alien finding us palatable?
- You'd have better chances of mating with a house plant than an alien using us for reproductive purposes. So that's out.
What we're left with is scientific study or reaching out for diplomatic relations. The scientific study could be a little scary since they would be much more advanced than us, and could consider us on the scale of lab mice...but even then they would most likely only take a few people.
Diplomacy would be a strong possibility if we had anything of value to offer them...that's not very likely.
This leaves us to the "zoo" theory---aliens know we're here, can watch us, but just aren't interested because we're too primitive.