There are effects that require target to make a Cha-based will save instead of Wis based.
Wis measures power of will, kind of like mental Con while Cha is more like mental Str.
When you try to influent others, like a mental brawl, you use your overwhelming charisma to win over others (Diplomacy, Bluff, etc.)
When you are mentally assaulted, what matters is your will power, how you protect your personality, not your personality itself. You can be both charismatic AND easily convinced, for example.
Is this better?