Why?

wtf.rb

class WTF
  attr_reader :color
  
  def initialize
    @color = 'red'
  end
  
  def color_with_self
    color = self.color
    color
  end
  
  def color_without_self
    color = color
    color
  end
end

require "test/unit"
include Test::Unit::Assertions

color_with_self = WTF.new.color_with_self
assert(color_with_self == "red", "#{color_with_self.inspect} should be 'red'")

color_without_self = WTF.new.color_without_self
assert(color_without_self == "red", "#{color_without_self.inspect} should be 'red'")