According to Elizabeth Landau of the Jet Propulsion Lab:
The Mars Helicopter (is) a proposed add-on to Mars rovers of the future that could potentially triple the distance these vehicles currently drive in a Martian day, and deliver a new level of visual information for choosing which sites to explore.
Scientists could also use the helicopter images to look for features for the rover to study in further detail. Another part of the helicopter’s job would be to check out the best places for the rover to collect key samples and rocks for a cache, which a next-generation rover could pick up later.
A proof-of-concept version of the Mars helicopter drone is currently in testing at the Jet Propulsion Lab. One big challenge NASA will face is getting the helicopter to fly on Mars, where the atmosphere is much thinner.