Archaeologist Anthony Swenson expounds reasonably on sheepherder's cairns. (Scroll to the bottom.) I had not heard the term Stone Johnnies before.
My father spent part of his Forest Service career riding the sheep range of the San Juan Mountains in SW Colorado, which is why if you asked him to say something in Spanish, he would reply,"¿Cuántas borregas tiene?"
He always offered the explanation, "They built them to have something to do."
The navigational-marker explanation makes sense in open country, but I have also seen the cairns in high mountain valleys where a person could not easily get lost without going over a ridge.
Of course, if you ask the modern-day shepherd why that cairn is there, he'll likely shrug and say 'don't know', but even if he doesn't know who built it or why, he still knows it's on the next ridge north of his camp. Thus, they continue to serve as landmarks, even if those who employ them as such have forgotten that they were built for that purpose. Which, when you think about it, is fairly delightful. How many man-made objects can you think of that continue to perform their intended function long after we've forgotten what that function was?