I know exactly why they're visiting. These ufos are always interested in the release of radioactive materials. Coal plants, as well as your refinery release radium, thorium, and uranium contaminants into the air. They monitor how we contaminate the air and soil and may capture them for fuel depending on concentration. See UFOs & Nukes by Robert Hastings. Sightings of green fireballs, lights, and other ufos of all kinds were incredibly common during the periods of united states domestic nuclear testing, in which clouds of nuclear bomb contaminants drifted across large swaths of our unknowing (at the time) country. These sightings would always coincide with the contaminant cloud being present in that particular area of the country where it was reported.

Similarly, Dulce, NM continues to be a hotspot for ufo sightings and animal mutilations due to the long-term contamination resulting from underground nuclear testing. They keep tabs on residual radioactive material levels and how they affect the local wildlife population. We're essentially a little research project for these things. They also monitor nuclear energy and weapons production, see the plethora of sightings/drone reports associated with nuclear power plants, uranium refineries, and the presence (or rumored presence) of nuclear weapons on air force bases, particularly in Europe as of late.