Like most of Marvel/Atlas titles of that time, this was an anthology of fantasy/science fiction/horror stories, usually with twist endings. This offering, however, was being put forward as somewhat more sophisticated than its fellow anthology series (hence the 'Adult' in the title), with writer/editor Stan Lee and artist Steve Ditko cranking out slightly more involved stories.
The title was slated to be cancelled after its fifteenth issue; so, with little to lose, Stan Lee used the opportunity to introduce a more offbeat superhero character (along the lines of the other superheroes that he was introducing into the Marvel universe, after years of anthology, Western, and teen-comedy titles). This of course was Spider-Man, and this issue (with the word Adult removed) would become one of the most famous and sought-after comics in history.
Without this final number, this would have been one of the more forgotten titles from this often-overlooked period of Marvel's history.