LL, I like a lot of these ideas. The ones offered so far.
In my setting Charisma is fundamental to Clerical characters. For sentence Charisma helps to determine how powerful the Charis, which are similar to the Charisms in the Medieval Player's Handbook. Similar, but not the same. For instance Charisma helps determine how influential a Cleric will be when trying to influence others, either emotionally, or reasonably (in debate) or how effective their homilies or preachings or speeches will be in influencing others. Charisma also helps clerics when practicing Thaumaturgy (miracle working) and Paladins when executing their powers, like
Lay on Hands. In other words Charisma helps to solidify and amplify Faith based acts and initiatives. Anytime one party or individual has to have "faith" in what someone else is doing then charisma helps to amplify this
"I have faith in you" proposition. Prayers become more effective, miracles more pronounced and obvious, effects more outstanding.
Likewise, although we don't use Charisma as Willpower, Charisma can amplify Willpower because it can help increase self-confidence in the character in the same way it can create faith in another.
In the clerical sense and in the faith sense then this is much more like the Koine (Greek) usage of the term. Charisma being a psychological or soul attribute of the character, rather than merely a social or behavioral one.
Charisma is also fundamental to Bards and to Agents and Scouts (Rogues) because of their jobs, for much the same reasons. Bards depend upon moving and convincing others, Agents and Scouts must instill faith in their proposals and work, and so charisma is considered a "type of genius" in these circumstances. Certain individuals and characters have the "genius of being charismatic" which goes far beyond being popular or social and into the realm of "deeply influential." Charisma can be used to win converts to your cause and to gain allies to asset you with your objectives. (And when you win a convert you can often reply upon their material as well as emotional support. Even if only in secret.)
Charisma is also extremely helpful for the human Wizard in my setting. The human Wizard is not a magic-user but rather a proto-scientist, alchemist, and inventor. Because he is usually "far ahead of his time" he often has to use charisma to help him convince others that his experiments, knowledge base, inventions, ideas, theories, and so forth are viable and workable.
Another use for Charisma is in ease of rising in rank. By this I do not mean charisma assists at the outset with social rank, but one with high charisma is more likely to rise in power and rank (I am not speaking about level) within a given organization. For instance suppose one is playing a Soldier. A soldier with high charisma and a good career record is more likely to rise in rank than one with poor charisma and a good service record. If they were equal in other respects then the Soldier with the good charisma is more likely to rise in rank faster. The same for a Cleric within his church, or anyone in any organization that relies upon social and political interaction. The "charismatic cleric" (and charismatic could imply far more than just "socially popular") is far more likely to rise to the level of Bishop or Arch-Bishop than the far less charismatic or retiring cleric. Then again if one rises far enough within certain organizations (the Church, the Army, the Government, etc.) then what usually follows is a de facto if not an outright public rise in actual social status and class. (By that I mean a rise in "Real Class," going from commoner to nobleman or important administrator or officer.) So in our setting charisma doesn't necessarily affect how you start out but can be extremely useful in how far you rise and what you eventually become (outside the mere rise in level for your professional class).
So charisma can have different and profession-specific effects, as well as more general effects, depending upon the situation and the given character.
Charisma in our setting is also a racial issue. Some races have more charismatic effects upon other races, and some races are just generally more charismatic than others, and all that implies.
We also use it to imply general good fortune in some circumstances.
And finally charisma can be very helpful to any class or profession when it comes to "leadership ability." Leadership is an important component of our setting. And charisma can be used to amplify or augment leadership ability and effects. (Or suppress the leadership ability of others. Individuals can engage in influence and charisma duels.)
Admittedly though
my setting is different from many settings in the way it approaches these matters because the intention is to make the setting far more like the real world of that time period than most, including things like Social Class and Culture being far more important than just a sort of generalized backdrop. I intend players to exploit their setting for advantage in the same way people can exploit the real world for advantage. Therefore things like rising in rank or becoming a representative of an organization are very important to character success.
But I do very much agree with those above who implied that it depends upon how the setting is structured as to how well any particular ability or capability can be exploited. How you structure attributes to act, react, or interact within the setting will determine how successful or important nay particular attribute is, or can become.
Don't know if my ideas helped you or not but good luck with your efforts.