Confused about when to use self again? This works the same without self. Also why use & instead :&

mapmethodmaker.rb

class Array
  def new_map
    new_array = []
    self.each do |item|
      new_array << yield(item)
    end
    new_array
  end
  
  def new_map!(&block)
    self.replace(.self.new_map(&block))
  end
end

Confused about when to use self again? This works the same without self. Also why use & instead :&