THE WHITE PAGE AND THE BLACK TYPEPAPER: The page the reader should see should be mostly clean, blank, and white (not yellow, blue, grey, or straw). Don't be fooled: white paper is the only paper a script appears on.
FONT: The type should be clear, dark against the white page, and very legible. The standard type face for a screenplay is Courier. Only Courier. And Courier 12 point (on a computer) or Courier PICA (on a typewriter). It is the easiest on the eye. It is the right size for proper formatting, and it is the only one you are allowed to use.
These two rules are probably the only two you should never break. And there is no reason to.
Like all of Standard Screenplay Format, the reasons for following it is that it is what the reader expects, wants, and will feel comfortable with. If you don't use the Standard Screenplay Format you are, obviously, not a professional screenwriter, you are an amateur, and your ability to write a great a screenplay is probably not as good as someone who knows both how to write and how to use something as simple as the Standard Screenplay Format.