Previously you could call functions by an expression of their name, e.g. "Call myVariable". In this case assume "myVariable" could be one of "foo", "bar" or "baz", and so it calls the function according to the provided string.
The new builtin functions don't support this. Instead you separately associate a string with the function. E.g. associate string "foo" with function Foo, associate string "bar" with function Bar, and associate string "baz" with function Baz.
Then you call a function by its string with the "Call mapped function" action. So you'd use "Call mapped function myVariable", and it calls either Foo, Bar or Baz based on whether the string is "foo", "bar" or "baz".
So it basically does the same thing as calling a function by an expression of its name, but with a different approach. If you want to pass parameters, it also has an option to forward parameters from another call. The benefit is now 'Find all references' is accurate, and you can safely rename functions without breaking anything.