why do we need ClassMethods and InstanceMethods?

I read the API for ActiveSupport::Concern. There are ClassMethods and InstanceMethods, we can put class methods in ClassMethods.

But the M's host can use the methods defined in M, can't it? Why can't I just write:

module M
  def self.x
  end

  def y
  end
end

rather than:

module M
  module ClassMethods
    def x
    end
  end
  module InstanceMethods
    def y
    end
  end
end