The idea is they aren't delivering to houses directly, they're moving packages between shipping facilities. So, they can presumably having landing pads on either side that aren't near power lines.
675 pounds isn't much, but if you have a high-priority delivery that's not very heavy it might make a lot of sense. Especially in remote places; they're particularly interested in serving places that are currently served by planes and helicopters, like remote locations in Alaska.
Yup, this is basically it. It's impossible to compete with a truck that's fully loaded, but there are tons of short middle mile trips where trucks aren't fully loaded (LTL or less-than-truckload freight). This type of freight is surprisingly inefficient, particularly in low density areas.
675 pounds isn't much, but if you have a high-priority delivery that's not very heavy it might make a lot of sense. Especially in remote places; they're particularly interested in serving places that are currently served by planes and helicopters, like remote locations in Alaska.