UAV mag surveying hits the sweet spot between ground-based and airborne magnetometer surveying. 

1. You are able to get the mag sensor much closer to the ground over traditional airborne mag
2. You can achieve a much line density over airborne mag (as little as 5-10 meter line spacing)
3. You can cover a much great area than ground-based mag surveys

So you solve the two primary disadvantages of airborne (usually helicopter) mag: You can fly lower and closer together. 

And you solve the major problem of ground-based mag: area coverage is limited to how far and where a person can walk.