I think what many commenters are missing is why this is important.

Invariants like this allow one to prove the correctness of their code. That may not seem like a big deal most of the time, but imagine you're working on a satellite that's going to Jupiter, where a bug in the code could mean 10 years from now a billion dollar project gets scuttled. Imagine you're building subroutines that will go into 10,000 pacemakers next year. Imagine you're writing the navigation logic for a cruise missile.

There are many times where proving your code correct is taken very, very seriously. Invariants are a tool to do so more easily.