It matters as soon as a requirement change comes in and you have to change something. Writing a dirty ass incomprehensible, but working piece of code is ok, as long as no one touches it again.

But as soon as code has to be reworked, worked on together by multiple people, or you just want to understand what you did 2 weeks earlier, code readability becomes important.

I like Uncle Bobs Clean Code (with a grain of salt) for a general idea of what such an approach to make code readable could look like. However, it is controversial and if overdone, can achieve the opposite. I like it as a starting point though.