The recommendation to keep lines widths less than 80 characters is a surprising part of the Python style guide (PEP 8). Really, 79 characters? Is it still the 1980's in Python-land? Maybe it's time to upgrade that monochrome CRT and get some screen real-estate.
Acting a bit on faith, and because my IDE kept squiggling my long lines, I decided to give it a shot.
Python places great stock in readability, so one motivation for < 80 is that wide lines are hard to read. That is absolutely true if you have to scroll horizontally, and probably generally true within certain ranges. After all, newspapers (yes, they still exist) are big but text is put into fairly narrow columns for readability.
The bigger win I've discovered is a subtle hint that maybe I'm doing something wrong. These things all become uncomfortable if you're trying to fit in a narrow space:
Of course, it can backfire a little, too, such as by encouraging overly short variable names when a better, longer one doesn't fit.
But when I start to run up against the margin, it makes me pause to think if maybe the code could be written in a better way. If so, great.
If not, a little wrapping is usually OK. Or just go long if it is really the best choice.