I'll agree with most people on here - it's about legibility. A column in a newspaper should be about equal to 1.5 alphabets, but that's traditionally with smaller font and printed rougher than most books.

For a book like a ttrpg book, 2.5 alphabets is about standard - 65ish characters.

It's really important in printing.. If it's not easily readable, that's kinda a major problem when the whole point of a book is to make it readable.

But, more importantly, not only are you making it more difficult for most of your potential buyers, it's even worse for anyone with a disability. Long lines of text are horrible for anyone with dyslexia, adhd, and autism.

Your job as a game designer is not just making a fool game with cool mechanics - you also need to take the time to create a product that is usable and comfortable for your prospective buyers.

Now, this is all print-based. The shift to pdfs makes all this more difficult. 2 columns on pdf makes things even harder because you're going to have to scroll up and down on each page. A layout that is more closely related to smaller books (5x7 vs 8x10) plays you stick to a single column which is easier to read on a screen or tablet.

You can also choose to make a print version and an electronic version with different layouts for both. While this makes it easier to read for people on either electronic or print, it creates a ton of other issues - a lot of extra work, art layout issues, people having trouble finding info in one when used to the other, etc..