1. With a wireless card you can move your PC around more conveniently.
2. Can replace the wireless card if it breaks or becomes out of date, rather than needing to replace the motherboard.
3. Very beneficial in apartments or that kind of setting where you may not physically have access to the router.
If you have your PC sitting in one place for a very long time and have your own router, there's not much reason to buy a wireless card or adapter.