Because it looks for errors from a small group of possible mistakes, pyflakes is very fast. When dealing with many lines of code this is a very useful feature. When used in conjunction with an IDE, it checks the code as it's being edited.
Pyflakes only checks for names that are defined but never used or names that are redefined without being used. It also does not have command line options that offer a degree of control for the tests that you are running.