The goal is to do this parsing exactly once, at the system boundary, and thereafter keep the already-parsed data in a box that has "This has already been parsed and we know it's correct" written on the outside, so that nothing internal needs to worry about that again. And the absolute best kind of box is a type, because it's pretty easy to enforce that the parser function is the only piece of code in the entire system that can create a value of that type, and as soon as you do this, that entire class of problems goes away.
This idea is of using types whose instances can only be created by parser functions is known as Parse, Don't Validate, and while it's possible and useful to apply the general idea in a dynamically typed language, you only get the "We know at compile time that this problem cannot exist" guarantee if you use types.
