When creating a function to run only one time, should the redefinition happen before or after the function?

I want a function to only run one time. I found this question here on SO and the highest voted examples redefine the function and then execute the code that is supposed to run once. I'm sure there is a good reason for doing that instead of running the code and then redefining the function but I can't figure out what that would be. My first instinct would be to only redefine the function after the code is run since that seems "safer" because the code would have to have run before being redefined.

Can someone explain why convention seems to be to redefine first?