You don't need a 5.1 speaker kit to listen to music, even though some sound cards can convert a stereo signal to a 5.1 signal. Very few music CDs offer multi-channel sound. While music DVDs offer the capability, they are hard to find.
However, most DVD films and many games use 5.1 sound, and for those applications, you need a 5.1 system for the full sound experience. Indeed, marketing hype aside, the surround sound experience requires speakers in front, behind and on the side, which necessitates at least 5.1 speakers. Software can widen and deepen sound, but it comes far from doing the trick compared to the 5.1 and above speakers.
Virtual Dolby Surround is an example of audio technology that purportedly enables two speakers to sound like five. But in practice, it's rarely as convincing as it is on paper - or in marketing brochures.