Take a look at their [link|http://www.rubycentral.com/ref/ref_c_file.html|File] class. Look at the class methods and the instance methods.
The class methods are all operations on the filesystem. The instance methods are all operations that you'd do to an individual file. If you see how they draw the distinction, you'll see why it can make sense to distinguish between them.
(The syntax for defining class methods in Ruby differs from Python, but the spirit is the same.)
Cheers,
Ben