Rolling dice for STR-DEX-INT-WIS-CON-CHR is the most emblematic and formative act in D&D. Maybe you've fought a melee or cast a spell or conversed with a dragon, but everyone who has ever sat down at the table for any length of time has picked up three six-siders and scribbled down the results.
The six attributes are such an oddly specific abstraction of a character's capabilities. They're the weird little numerical window you peer through when trying to figure out who your character is—who you are—in this shared fantasy. When I was younger, I would obsess over getting the "right" attribute scores, and this frequently tipped over into fudging. I played a statistically improbable number of half-elves with 18's in Dexterity and Charisma—I just wanted to be pretty, dammit!
And yet... what good are they?
The attributes are all there, fully formed, at 0e, but they seem to have arrived well in advance of any real use for them. There are a couple bonuses and the XP bump from your Prime Requisite. In Holmes, Strength and Wisdom have no function whatsoever except as Prime Requisites.
In the first issue of The Dragon, there is an article called "How to Use Non-Prime-Requisite Character Attributes." It begins,
Whenever a player performs a non-ordinary task, or attempts to do so, the referee is usually in a quandary — how to determine fairly whether the character can perform the attempted action? Normally, the referee gives consideration to the player’s attributes and then more or less ‘wings it,’ attempting to be fair — usually giving the player a percentage chance of success.What follows is a pretty entertainingly convoluted process that involves rolling a die to determine which die you use to multiply by the attribute score in order to arrive at a percentage which you can then attempt to roll under.
Then Moldvay spelled out the almost irresistibly elegant "There's Always a Chance" rule, albeit as little more than an optional footnote. And over the editions, more derived stats and bonuses were accrued to the point where a common grognard complaint is about having de-emphasize attributes.
All of which makes the attributes look like a mechanic in search of a function.
So why were they there in the first place? Were they character-development fluff? Was there crunch that I'm missing? Or was this an arbitrary collection of seat-of-the-pants rulings (as I assume the saving throws were) that fortuitously became a centerpiece of the game?
EDIT: Some excellent answers in the comments. tl;dr: Time to go buy a copy of Playing at the World.