Doesn't seem to be a common thing to pay attention to outside of graphic/UI/UX design world, but if you stare at the code for significant chunk of the day, it's a good thing to customize/optimize at least a bit.
I've always used var-width fonts for code in emacs, and like them vs monospace ones for general readability and being much more compact, but noticed one major shortcoming over the years: some punctuation marks are very hard to tell apart.
While this is not an issue in free-form text, where you don't care much whether some tiny dot is a comma or period, it's essential to differentiate these in code.
And in fonts which I tend to use (like Luxi Sans or Liberation Sans), "." and "," in particular tend to differ by something like 1-2 on-screen pixels, which is bad, as I've noticed straining to distinguish the two sometimes, or putting one instead of another via typo and not noticing, because it's hard to notice.
It's a kind of thing that's like a thorn that always torments, but easy to fix once you identify it as a problem and actually look into it.
Emacs in particular allows to replace one char with another visually:
(unless standard-display-table (setq standard-display-table (make-display-table))) (aset standard-display-table ?, (vconcat "˾")) (aset standard-display-table ?. (vconcat "❟"))
Most fonts have ton of never-used-in-code unicode chars to choose distinctive replacements from, which are easy to browse via gucharmap or such.
One problem can be emacs using faces with different font somewhere after such replacement, which might not have these chars in them, so will garble these, but that's rare and also easy to fix (via e.g. custom-set-faces).
Another notable (original) use of this trick - "visual tabs":
(aset standard-display-table ?\t (vconcat "˙ "))
I.e. marking each "tab" character by a small dot, which helps a lot with telling apart indentation levels, esp. in langs like python where it matters just as much as commas vs periods.
Recently also wanted to extend this to colons and semicolons, which are just as hard to tell apart as these dots (: vs ;), but replacing them with something weird everywhere seem to be more jarring, and there's a proper fix for all of these issues - edit the glyphs in the font directly.
fontforge is a common app for that, and for same-component ".:,;" chars there's an easy way to scale them, copy-paste parts between glyphs, positioning them precisely at the same levels.
Scaling outlines by e.g. 200% makes it easy to tell them apart by itself, but I've also found it useful to make a dot slightly horizontally stretched, while leaving comma vertical - eye immediately latches onto this distinction, unlike with just "dot" vs "dot plus a tiny beard".
It's definitely less of an issue in monospace fonts, and there seem to be a large selection of "coding fonts" optimized for legibility, but still worth remembering that glyphs in these are not immutable at all - you're not stuck with whatever aesthetics-vs-legibility trade-offs their creators made for all chars, and can easily customize them according to your own issues and needs, same as with editor, shell/terminal, browser or desktop environemnt.