There are two reasons why your employer requires you to fill in the form.
The first is that if you're off for 4 days or more, you'll automatically be entitled to Statutory Sick Pay. This is paid, through your employer, by the government. So, you'll still get this even if your employer doesn't pay sick pay himself. (It's only �68.20 a week but it's better than nothing).
The second is that, by submitting a self-certification form, you'll be providing them with a document which could be used in disciplinary proceedings if the Match of the Day cameras happen to have pictured you supporting your local team when you claimed to be too sick to leave your bed :-)
Chris