Because x(a) -> b might need to be understood by everyone working with your module at some point. What if you leave the company and someone inherits your module

Because x(a) -> b might need to be understood by everyone working with your module at some point. What if you leave the company and someone inherits your module? What if they decide to change their codebase and need to rewrite your module? Writing modular code doesn't magically give you maintainable code either. It needs documentation, proper use of design patterns, and needs to be readable.