It is implied in Battle: Los Angeles that the aliens have come to Earth to plunder it for its resources, specifically, its water.
We often hear about how Spaceship Earth is a unique little snowflake in the big bad universe, what with the presence of life and liquid water, but that doesn't mean that there isn't any water anywhere else. In fact, it is one of the most abundant substances in the universe.
In fact again, the one thing that does give the Earth its Special Snowflake status (although the existence of the invading aliens rather diminishes that status) is the thing that prevents the aliens doing exactly what they want: Humans.
If they were looking for resources, they could have found them in abundance and undefended in the rest of our solar system. Hell, Jupiter's moon Europa is thought to harbour more water than all of the Earth's oceans combined beneath its icy surface. Alternatively, they could mine it in frozen form, along with huge amount of other precious elements, from the vast asteroid belt or vast Oort Cloud.
If they were looking for somewhere to live, it is difficult to believe that these system-hopping alien beings could have found a nice, habitable exoplanet anywhere else in the galaxy. Preferably one that isn't already bristling with guns.