Many, if not most, programmers leave word wrap off by default when editing code. This is because line-breaks in code can have programmatic implications. Python, for instance, uses line-breaks to indicate the end of a statement. With word wrap enabled, coders may have a difficult time distinguishing between new lines created automatically by word wrap and new lines created intentionally by a carriage return.
That’s all well and fine when you’re working with actual code, but it doesn’t work so well with documentation meant to be written in plain English (or some other traditional language). On one hand, s single long horizontal line of text isn’t very readable — newspapers invented columns for a reason — and horizontal scrolling is a pain. On the other hand, manually inserting line-breaks into text isn’t much better. Adding or removing just one word at the beginning can require deleting and re-typing all of the newlines in a paragraph. And because documentation often lives in the same file as code (i.e. as comments), most text editors apply the same word-wrapping scheme to both code and documentation.
It also doubles as a way to convert fixed width multi-line text back to — e.g. the angle bracket quoted paragraphs in e-mails — back to a traditional word-wrapped single line of text.