Every so often you would have to change out the mainline and the haulback wire rope for new lines. The Linehorse is what brought out your new wire rope to your machine and took the old ones back to the yard.
That old Sproat linehorse in the pics has a purpose built Promac Linehorse winch, somewhere I have a brochure from Promac that shows it. I doubt that they sold many of them, most camps had any old winch bolted onto the most burnt out truck on the claim. I even saw a picture once, it was a 10-10 Lawrence in the back of an old rock truck. Hopefully it was bolted down! CZ in Sandspit had an HDX with one huge drum powered by a Ford gas engine.
The linehorse evolved from the rig-up goat used in the last days of the wooden spar trees. The goat would go out with the bull gang and raise the trees, string the guylines, etc. Later, a logging truck would deliver the yarder, and the tree would already be set up for them; just thread the lines and start logging.
Of course, seeing as the goat was just a truck mounted donkey, it had many other uses. Righting overturned machinery, pulling trucks up steep grades, etc. They were very handy machines. Lots of guys were upset when they replaced J-91 with an F-700 Ford. All that thing was good for was transporting a spool of line.