For the same reasons you'd want to use italics in a proportional font - for emphasis or contrast.
1. Syntax highlighting which uses both color and font style, e.g. class name are italicised by default in my editor (Sublime Text).
2. Editing markdown/HTML files with italics tags/markup.
3. Also, italics also work on the console, and some console/TUI apps may use it.
It started because I'm picky when it comes to colour schemes. I'd find one or two things that annoy me and I don't personally have the imagination to fix it, so I'll try to find something else. Or I would get bored of a particular colour scheme and go hunting for something new and fresh.
Removing it took a bit to get used to but now I can't stand going back.
I don't personally subscribe to the Rob Pike idea that syntax highlighting is for babies or whatever, to each their own.
I’m with you that colors can easily start to impede readability. I’m a fan of subtle, judicious color use, so I’ll often start with a palette but knock back the colors from there.
What typeface(s) do you like for its italics?
I used Victor Mono for a bit there and I'm still unsure if I like cursive italics or not. They're interesting and fun, but sometimes I think they're a bit too out there, if that makes sense?
My go-to is usually Ubuntu Mono. That might be my favourite font.
If you want monospace for the crisp, clear, draft-style, clutter-free look when writing code (or similar) then it's excellent. If you absolutely must have every character being exactly the same width then less so as the main difference is that it is monospace for everything except 'm', 'M', 'w', and 'W' which are each 1.5 wide (so it takes two to bring things back in alignment). In practice that is often less of an issue than it seems, and the benefit in readability/clarity/looks (literal mono for those characters makes them compressed) makes it worth giving it a try, even if you only end up using it for Markdown files etc.
They have a web page explaining it (alongside a download): https://ia.net/topics/in-search-of-the-perfect-writing-font
I find color themes with more than one or two front face hues to be tiring to look at.