Besides the points already mentioned by shadowgovt, another big advantage of drones over existing modes of delivery even if drones have to follow existing streets is that they can be self-flying fairly easily.

Unlike self-driving cars, self-flying drones do not have to worry about people running into their path, cars parked in the parking lane adjacent to their line suddenly opening a door into their line, or sharing the road with unpredictable human driven vehicles.

The drone route regulations will be starting with routes that have no other traffic except birds and insects. The routes can be designed so that there are different levels for north, south, east, and west travel, so except at the endpoints a drone never has to worry about encountering another drone moving in a different direction. It only has to deal with drones coming up from behind or coming up on drones ahead of it.