It's not. The copyright year shows when the content was first published (if a range, then when it was last modified). If the year is dynamic, then it's untrustworthy.

If I'm reading some content in the year 4023 that says (C) 2023 CM30, then I know the copyright has since expired and I'm free to do what I want with it. If it's dynamically saying (C) 4023 CM30 then it doesn't say when it was published.

If I'm publishing something in 2019 with a dynamic date, and it says (C) 2023 and by astonishing coincidence it's very similar to another piece of work by another author that has a static date and it says (C) 2021, it appears that my work post-dates their work rather than pre-dates it.

Mostly this matters much less now and there's other ways of proving publication date but in older media it mattered more.