You have to have been around for awhile to know the history and why it is called a nail knot. The most famous fly-fishing writer of the 1960s and much of the 1970s was Joe Brooks, who pioneered fishing in remote places and bringing new techniques to the attention of his readers.
Brooks targeted large trout in Argentina when few knew such wonderful fishing existed. On one of his trips in the 1950s he was shown a new method of attaching the butt section of tippet to the fly line. Argentine fly-fishermen used a horseshoe nail, which is flat and slightly tapered, to form the knot. The horseshoe nail was held against the end of the fly line and the butt section, then wrapped around both. The tapered nail was extracted, allowing the tag end to be slipped under the coils before tightening; hence the name "nail knot." Brooks wrote about this in Outdoor Life magazine, and within a short time fly-fishermen realized that using a tube was much better than using a nail.