The idea was to be able to haul short trailers more efficiently. In the 60s, the longest standard trailers were 40 ft, so a hitch on one end, and one in the middle, would hold two trailers. If one wanted to carry shorter trailers, or the new containers-on-chassis, it would still only hold two.
By adding two more hitches, it could carry three short trailers, one at each end, and one in the middle.
Back then, all hitches had to fold down, so trailers could be backed over them, and only the ones needed would be raised, after that trailer was in place. They had to park the trailer, pull the tractor out of the way, then raise the hitch. Tricky, as the trailer kingpin had to be in exactly the right spot to fit the slot in the hitch fifth wheel. If it wasn't, they had to lower the hitch, respot the trailer, and try again.
