You have spent three months working on your coursework. It is absolutely brilliant, even if you do say so yourself, you just know that it is worth that A* grade. How would you feel if you found out someone in your class had secretly copied your work when you went out of the room for five minutes and then handed it in as their own? I bet you would feel pretty miffed, to put it mildly.
How about that song that you spent last week writing? You are sure it is great chart material and is going to get you that break into the music industry. You record a demo of it and send it to a record company, but you don't hear anything from them. How does it make you feel when you see the latest boy band performing your song on Top of the Pops, and you haven't been paid a single penny for it?
It's not fair is it? That's why we need a law to protect the author of any original material from someone else taking it and claiming it as their work.
There is such a law, it is called the 'Copyright, Designs and Patents Act' and it was passed in 1988.