I will not get into the legalities, ethics or efficacy of bootlegs, nor the definitional aspects (that is, what is a "bootleg" and how it differs from "counterfeit" and so on).
"Great White Wonder", which appeared in the late summer of 1969 in the USA, is reckoned to be the first ROCK bootleg album but it was not the first example of bootlegging recorded music.
Over 100 years ago, a guy called Mapleson, who had been given a recorder by Edison and was the librarian at the Metropolitan Opera Hose in New York, would record brief snatches of performances in the venue. The Mapleson Cylinders, albeit individually brief in content and poor in sound quality, are now regarded as of historical importance. In the years after WW2 and into the 1950s, commercial bootlegging began to grow - classical music, operas, jazz, blues - taken from radio broadcasts, live performances, old 78s not available on LP, "original soundtracks" from films that were not available officially, and so on.
Many of the early private recordings of Dylan were not made with a future commercial value in mind. Individuals simply made recordings for their own use, for playing to friends or for copying for friends. Dubs of studio recordings were made for similar reasons. Sometimes, acetates of those recordings were passed around to encourage other singers to record the songs. Copies of copies of all of these were passed around between fans, anxious to hear "everything" (or as much as possible) recorded by their favourite artist. In more recent years, it appears that concerts are recorded by bootleggers, on top-notch equipment, with a view, from the outset, of making and selling bootleg CDs.
Finally (and I hope I haven't said this before), it is not unknown for big record companies to bootleg competitors' output - not as a whole but in part, at least. I can't find my note on the subject but, as I recall, Columbia Records discovered, after the event, that they had failed to record the whole of one orchestral piece. These recordings, when made in the studio, are done in sections and then the various sections are joined together to make the final record - and they found they had a gap! Since it would have been almost impossible to bring everyone together again to record the missing section (and, doubtless, disproportionately expensive, too), their solution was to "borrow" (for which read, "steal", "misappropriate", "nick" etc) the missing part from a recording made by someone else for another record company. Not aprevalent practice, I'm sure, but if it happened once, it may well have happened on other occasions.