Sorcerers use Charisma as their primary Ability Score for spell-casting in Dungeons and Dragons. Wizards use Intelligence to cast, and Clerics use Wisdom. Intelligence and Wisdom make sense for the respective character classes, both mechanically and flavor-wise.
Why do Sorcerers use Charisma? What is the reasoning from a mechanical, a class flavor, and/or D&D historical sense?
My first encounter with a D&D sorcerer class was 3E. The 3E PHB says on p51:
Sorcerers create magic the way a poet creates poems, with inborn talent honed by practice. They have no books, no mentors, no theories -- just raw power that they direct at will.
In religious studies, "charisma" sometimes refers to the inner personal power in an individual, divinely conferred. I had to read endless writings about it in college, especially Max Weber, who described it as:
Charisma is a certain quality of a individual by virtue of which he is set apart from ordinary men and treated as endowed with supernatural, superhuman, or at least specifically exceptional powers or qualities. These ... are not accessible to the ordinary person, but are regarded as of divine origin or as exemplary, and on the basis of them the individual concerned is treated as a leader.
This is the aspect of Charisma often underplayed. (Also, I am not suggesting that anybody involved in writing D&D had to have read Weber, only that he's a good source for reading about this aspect of it.)
Charisma isn't just good looks or even greatness of personality. There is a sense of transcendent greatness about highly charismatic people.
You'll find more references to this sort of meaning of Charisma on the Wikipedia page for Charisma.
Mechanically, if the game's mechanics center around combat, it's good for all stats to be useful for that purpose. Otherwise you get dump stats that nobody wants, and you may as well not have the stat at all and just let people roleplay that aspect of their characters however they want.
In D&D 4e, they continued this trend with the wider spread of primary character stats (although this made the stats themselves less distinct). It's possible to find a class that's good for almost any two main stats in the particular role you want to play, which makes it easier to design a character with particular strengths and weaknesses in mind and who can still be useful in a fight.
As far as I know, many other RPGs don't have this problem because they aren't oriented around killing monsters the same way that D&D is, but other systems will still have forms of spellcasting based on force of personality because it's flavorful.
I generally look at charisma as related to emotion. Think of raven from teen titans her powers are natural and ruled by her emotions. As a master of emotion you also tend to be a master with dealing with people. So I believe that charisma based casting is related to emotion which makes perfect sense to me, as sorcerers have natural powers gifted to them from birth which are ruled buy their emotions. Also people fail to realize that the charisma based skills are incredibly powerful. You can damn near alter reality with a good bluff. You can become rich beyond all reason with diplomacy as well as save your ass from sticky situations. You can also scare the crap out of people. The only useful encounter skills in the game are charisma based.
Sorcerers use Charisma, because later on in 2e's life people weren't using NPC reaction tables or henchmen very much, so its original purpose dropped off and everything was almost entirely roleplaying. It had to have some purpose to avoid being a dump stat for everyone, so Sorcerers were powered off Charisma simply for the sake of giving Charisma something important to do.
