Off-loads sample mixing to the sound card. Computers of that era were slow enough that doing 44100 additions per second was expensive (IIRC quake had 8Khz samples anyway).

Something that always seems to surprise me when I remember it today: Playing an MP3 on my Pentium CPU in 1996 worked, but it _really_ brought the system to its knees. I could choose between listening to MP3 music or interacting with some other program (mIRC, Netscape, etc) but certainly not both.