They are here for three reasons.
1. To free this planet and it's citizens from the control of the Annunaki. They're not being sneaky (if they were you would never know they were there, and thus not hear the sounds), they're simply preparing for the battle. However, the battle will not be Humans vs. Aliens but Annunaki vs. Aliens - the incoming force has no desire to conquer or destroy humanity.

2. To restore this planet to it's former glory.

3. And last but not least, to welcome humankind into the Galactic Community.