Why does very single possible opening need a name?

Idk

Chess has an incredibly long history, with a gigantic literature behind it. Over the years, certain openings and variations have been named either for the master who first played or analyzed them, or for the location or tournament where they were first played. 
Take the Ruy Lopez, for instance, named for the Spanish priest who first wrote about it. Naturally, it's also called the Spanish opening. But over the years, as it became one of the most popular lines, it was played and analyzed thousands of times, and the most popular variations in the line have also gotten names. The Morphy defense, the Berlin defense, and (I believe) the Marshall defense. Each has its merits, and each then leads into dozens of complicated lines and middlegame positions. 
Some openings, too, have rather fanciful nicknames such as the Orangutan opening or the Fried Liver attack. But, as others have said, chess is far more than merely memorizing the names. For the masters and above, the name of the opening and its variation is simply a shorthand for them to communicate quickly to each other just what they played in a given game. When one says to another, i played a King's Indian, a picture immediately comes to mind as to what the board looked like--much quicker than reciting a series of moves. Kind of like the titles of movies and books. Imagine having to tell a friend that you just read "To Kill a Mockingbird" without it's having a title. How would you go about that?

Imagine looking for a (classical) piece without knowing its name. Even if you know the first few notes, it's very hard to find it.