What are the biggest differences between On board sound and a Sound Blaster Audigy sound card?

Right now I use onboard and I plan to upgrade to the above card but I'm just curious because right now I'm a little strapped for cash (and currently seeking a second job). When I play music files or CDs the sound seems just great. I'm not a big time Computer gamer either.

Where will I be getting my advatages with the sound card? Is it worth the $80 I plan to pay for it, or should I just keep my on board for now?

Thanks guys