If you were touring and needed to tighten your crank arms, it just wasn't practical to carry a ratchet and socket. Remember, life on the road was a minimalist experience and all that stuff is just to heavy.
This little tool served two purposes. It was of course, a bicycle tool used to tighten a loose crank arm
preferably, to maintain tightness on a well used bicycle, like a fully loaded touring bicycle.
But it was also a utensil used to spread peanut butter.
PEANUT BUTTER!?? Why not cream cheese, jelly or caviar? Well, it would work for that too, but when you are on a cross country tour, and you are carrying everything you need to sustain yourself for days on end, and enough equipment to last the duration of the your tour, weight was an issue. Refrigeration was a problem.
For example, If you bought a meat that was frozen, you'd plan your meal around the thawing time. When all that was perishable was consumed, peanut butter was an excellent source of calories, it was compact, loaded with protein, calories and did not spoil and was much less expensive than caviar! Yes, they carried jelly, but peanut butter would challenge plastic utensils and many of the steel camping ones as well. One day a resourceful cyclust (a redundant trem) had enough of bent campers flatware and fished the perfect tool out of his tool kit. I presume he washed it, or at least wiped it off, and thus began a new tradition. Word spread like a California Wildfire in the bicycle touring community.
If you notice the markings on this particular tool it reads::
PARK TOOL CCW15
It's a Park Tool and it's a Crank Wrench 15mm-
Perhaps it should be re-named the "PBT15"
so it earned itís dubious name.