You can often back out if you can drive in; sometimes, however, you either cannot do so in a timely fashion, or you cannot do so at all.
In the picture you posted, backing is possible; likewise, the drop-and-pick maneuver depicted is a very good option, because the room to do either option is present. Now, refer to the picture I posted: 30" of water, a solid track about 7' wide, and totally unknown terrain ahead and to either side. You can neither back out of that road, nor can you drop, maneuver and re-hitch a trailer; you can drag it in, but until you hit a clear patch that allows maneuvering, you cannot maneuver the trailer in any other way.
So, let's be accurate: backing is sometimes possible, and maneuvering is sometimes possible. At other times, neither can be accomplished; as a result, unknown roads - and what I posted is about as bad as it normally gets - are no place for trailers, and they should not be taken into those areas.