Ruby modules and extend self

In what sort of situation is the code:

module M
   extend self
   def greet
    puts "hello"
   end

end

more beneficial to use over say something like:

module M
   def self.greet
    puts "hello"
   end
end

In the top, one is an instance method being extended, and the latter is just a class method, but when calling either method, you'd have to M.greet , right? I was just curious if anyone could shed some light on when to use one code over the other. Thanks!