First off, it doesn't increase performance. And secondly, if you have shitty speakers, there is no point. If you have really good speakers, i'm talking good as in a brand name, a large sub, and a quality surround sound setup with a lot of power AND you are a person who is picky about sound then a sound card is fine. I have a 700watt Amp with a 5.1 channel surround sound in my room, My front speakers are 3-way with 12'' woofers each and a powered 10'' downfiring subwoofer by JBL and my onboard soundcard that comes with my Asus P6T gives outstanding sound, both music and games.
I am not the pickiest sound person, i just love massive bass.