Fonts for Writing
Often, you don’t get to make a choice of fonts on your computer. You’re browsing someone else’s web page, viewing a PDF, or being asked to collaborate on a word-processor document.
Computer programmers work on code to be read by other programmers, but the
font, the formatting, the size, are not sent along with the code. We as a
profession are free to choose how (most) text looks on our screens. As a matter
of tradition, most programmers choose monospaced fonts , such as you would
produce with a typewriter.
I don’t buy most of the reasoning I’ve heard to justify that you should only code in monospaced fonts, so for some time I’ve used the Verdana font for programming, and it works just fine.
Last week I read a post with a proper justification to choose a monospaced font over, say, Verdana. 
When you need to edit text, monospace has a major advantage over variable spaced fonts: it’s easier to see and select narrow letters like “l” and “i”.
I had not considered this, and yet I have suffered it since I use modern personal computers: often, needing to replace a block of text, “just left of that i”, I have struggled to select only the part I wanted with the mouse.
So, I suppose monospaced fonts are a good choice for anybody doing lots of
editing. The iA Writer application is one of several
tools that are optimized for writers, and that use monospaced fonts.
Some people swear by them. For people who have known only the complexity of MS Word and its ilk, these applications can be a revelation.
The reasoning that usability requires monospaced fonts, though, is not complete. You could have variable-width fonts with i’s, l’s and punctuation wide enough to handle comfortably. That would make for a good writing font, too.
There are several fonts that tackle just this problem. The people behind the aforementioned iA Writer have developed a “duospace” font, which is basically a monospaced font where m’s and w’s are given 50% extra width. 
There is recently some work on monospaced-inspired variable fonts . I’ve tried a few of these, but taste is a tricky thing, and I haven’t particularly taken to the ones I tried.
Against monospaced fonts: they’re not as comfortable for reading as variable-width fonts. Of the variable-width fonts, some are quite good for editing. I stand by Verdana being a very good choice for both writing and reading.
1: wikipedia on monospaced fonts
2: the reason to choose a monospaced font