Why do we use the "class << self" expression?

In Ruby there are several ways to declare a class method.

We have these two forms...

class Dog
 
   def self.species
     puts "I'm a dog."
   end
 
   def Dog.species_rly?
     puts "Still a dog."
   end
 
 end
 

And this other, longer form...

class Dog
   class << self
     def species_srsly?
       puts "OK, fine. I'm a cat."
     end
   end
 end
 

Why is the last form used? How is it better than just doing this?

class Dog
   def Dog.some_method
     # ...
   end
 end
 

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