Fixed-width fonts allow for easy tabular rendering in a text editor. Some programming styles use this heavily but it’s not universal. Particularly, if you rely on line-by-line blame in source control, alignment changes can make it harder to find the commit where the most recent semantic change happened.
I find type writer fonts to look horrible since I’ve gotten used to variable width fonts. It’s like immediate eye strain for me. But you have to choose a good font, times new Roman won’t work.
I wish I had more specific examples and could quantify it better, but I know I've caught quite a few errors this way.
If you're using tabs for indentation, and not trying to do pointless alignment then there's really no reason to use monospaced fonts at all - it just makes things uglier.