IMO, validators are needed, when a function caller is unknown. What are the scenarios where the caller of function is unknown?

• When the function is called by users of the application.
• When the function is called by users(other programmers) of the module that has the function.
• When the function is called the author of the function, who happen to forgot what are all the parameters that the function takes.

So,

• If I am the only person who would ever call that function AND
• If I am sure that I would remember the parameters or I would always ensure that I pass there right parameters:
  • I wouldn't write any validations.

If the same function happen to be used by other programmers or users, the first thing that I would do is to add validations.

When I am adding validations,

• If I am sure that the function users are programmers, then I would simply have assert statements or throw statements.
• If the users are functions are actual users, then I would have human readable error messages.