If your current motherboard on your desktop PC has a bad sounding onboard sound, using a seperate PCI sound card can improve the sound quality. The sound cards' sound processor unit may help make your games run a few frames faster because the sound card has its own sound processing chip and RAM, so processing sound may rely less on the CPU to decode sound.
Having a separate sound card maybe useful for Virtual Reality/VR where many users may listen to sound through expensive surround sound headphones, or a nice surround sound speaker system. Having bad, and low quality sound can make VR less realistic which makes the VR game less entertaining. Sound, and video slowdown problems where the sound is slower than the video, or the video is slower than the sound can, and they are not both running at the same speed is annoying.
The extra few frames per second from using a separate sound card with its own sound processor, and RAM can make games feel more smooth in VR.
