They're unnecessary, but mainly because tractors have equipment that do this anyway. So unless it's needed more frequently than when the tractor's driving around, it's probably not worth the extra cost.
I've seen a drone scanning over a large corn field, dropping sensors seems silly when you can outfit infrared tempature sensors/humidity sensors/etc on a drone.
I thought it was till, plant, harvest, and in between you just wait for nature to take its course. Do they have occasion to patrol a field with tractors more often than that?
On some crops yes, you spray all kinds of things after plating. On other crops you don't dare passing a tractor around them, else they would be ruined.