Answer: So that it can put shingles on your roof for you.
While it's much less terrifying than a drone with a flamethrower, a drone with a nail gun should still be treated with a heavy dose of caution. It could, however, be a boon for construction workers, for whom the task of attaching shingles to a roof is both monotonous and dangerous at the same time. Sounds like an excellent task for an autonomous machine.
A team of roboticists at the University of Michigan developed a prototype drone that can attach shingles to a roof. Using cameras and markers placed around an area, the drone can find the exact spot to put each shingle, and then secure it in place with an onboard nail gun.
The tech is currently just a demonstration, so the drone can only stay in the air for 10 minutes and put in a few dozen nails. With further development, however, it could become a very useful tool for building.