He had senile dementia and liked to go outside naked, but he could still do two things perfectly: win at checkers and write out prescriptions.
Suppose that throughout your childhood you were good with numbers. Other kids used to copy your homework. You figured store discounts faster than your parents. People came to you for help with such things. So you took accounting and eventually became a tax auditor for the IRS. What an embarrassing job, right? You feel you should be writing poetry or doing aviation mechanics or whatever. But then you realize that tax collecting can be a calling too.