Pretty much any motherboard has built-in audio, and in modern times its passable quality compared to what used to come onboard. So no, you dont "need" one. If your an audiophile and would appreciate the extra quality from a dedicated card, I suspect you would already know the answer to your question
.
Myself, I use an ASUS Xonar DGX sound card. I haven't noticed any increase in quality
per se, but I definitely have noticed the head-phone amp. Having more power pumped to my headset means that it can go louder and has the power to drive the more bass sounds without getting crackling. If your using a decent headset/headphones, I recommend you look into a cheap $20-30 sound card, if only just for the amp.