So why code in a monospaced font then? There's no rule that says you have to write code in a monospaced font. I've used proportional fonts for many years, and almost every programmer's editor supports them.
I'm currently using a slightly modified version of Trebuchet MS (I replaced the tilde with a better one). Another example is Input Sans, one of the three fonts today's thread is about.
If you like proportional fonts, go for it!
All respect to those who like proportional fonts, to each their own, but I think of code as a 2d grid.
Same reason I use syntax highlighting: color is another dimension that helps me see what I’m doing. This is particularly notable in Lisps, where turning on rainbow parens can be like switching on a light switch, immediately helping me understand the shape of the forms.
I have never really liked any of the other popular coding fonts and it really comes down to one thing: They are all stupid sans-serif fonts.
I have found, ever since displays grew out of the garbage 100 dpi age, that serif fonts are vastly more enjoyable to look at and easier to quickly visually discern.
As a result, Iosevka Slab is basically the only code-optimized font I can even consider.
Beyond that, I prefer a narrower font for monospace text. Monospace is already such an awful layout for readability, but, making it narrower at least helps reduce the horizontal scan range without imposing insane limits like 72 character lines.
So, those are my reasons anyway.