Regular people are not often put into conservatorship because it isn't worth it to spend $100k in legal fees to protect less than $100k in assets. People in true crisis rarely have large amounts of money anyway. That's sorta the beginning and the end of the explanation. Rich + mentally incapacitated is a rare combination.
The idea is for people who care to preserve some of the person's assets so that when they get well or get clean they won't be at financial rock bottom. And in the meantime there's money to pay for their care.
I would be interested to know if anyone has studied how often this narrative plays out, versus the Britney Spears narrative, and versus the vulnerable-person-fleeced-of-everything scenarios.
If you are an alcoholic, drug addict, senile elder, or mentally ill person, the circle of vultures forms quickly around you. You have already lost your freedom anyway, and can only act in ways detrimental to your well-being, so the only question is which business or person will take how much of your assets. None of these conditions are anyone's fault necessarily (let he who has never tried drugs or alcohol, and who is immune from mental illness cast the first stone) but it is a feature of our system that anyone making consistently sub-optimal decisions will eventually be picked clean, financially.
Conservatorship can be used to put control in the hands of just another vulture, but it hypothetically allows families to not be crushed by the costs of caring for a relative out of pocket, after that relative has lost their wealth to the vultures. Cher, for example, will end up paying for her child's care after he spends all his money on drugs. He is a liability on her books, so she has a claim on his assets for causing this liability.
