Because the artist requires it.

Websites exist to serve content. The author of this page demanded it have a background image.

Here's another page that doesn't:

https://hypertele.fi/b40bfc44ff8a22df

That subtle background gradient is lovely. I like how it adds a frame to the text without distracting from it. It evokes the right vibe, I think.

That's a design element that serves the functionality well and doesn't feel superfluous at all. But the image on the colophon page I'm less excited about. It gets in the way and feels unnecessary.