VLC is media player so a person with media might use VLC. Normal average user might have video files originated from her camera and then she wants to play these on computer. This computer runs maybe Windows or OSX. In this average case VLC can act as an alternative video player. So my question is really why an alternative video player?
VLC is also the core video player in Linux. So obviously VLC is significant in that market. But VLC is pushed to all users. Why? One reason might be easiness. Development business for Linux can be easily transfered to Windows. This is just my guess.