It's a pretty common width for wrapping, especially in Markdown. I know, for example, when reviewing other people's Rmd and Qmd on GitHub, it's very helpful to be able to comment on lines separated either by width or by sentence, rather than have a comment connected to a whole paragraph.
I know in terms of accessible web copy, the optimal line length for reading for the human eye is between 50 and 75 characters, but that's about reading (and I don't think the research was done with coding/monospaced fonts).
Obviously, you can turn off the margin line, or change the width to whatever you prefer. I found this one post from someone who's clearly a fan of 80-character width:
Edit Apparently wrapping is a contentious topic. If you're curious, I found the post below interesting: