fixed-width monospace fonts
Why Do Programmers Use Courier Typeface?
Courier is just one of many monospace fonts. They are also called fixed-width fonts. Consolas is the default font in Visual Studio, and there are even better fonts for programmers.
We like fonts where:
0 cannot be confused with O
Punctuation characters like "," are bigger because they are far more important in programming than in daily use
Brackets are distinct
"1," "I," "l," and "|" cannot be confused (that is 1, i, L, and the pipe sign)
This leads to fewer bugs. Mistyping "," as "." will often break your code and at least lead to unexpected behavior. The same is true for ":" versus ";" and so on.
Slate explainer rides on Quora.