Why multiple soundtracks?

[AWaite]

Why do DVDs usually have stereo AND DD5.1 soundtracks? I've always thought that maybe some older, or computer-based players, may not handle auto conversion of DD5.1 to stereo. But I've just noticed that Traffic (Region 2) has only a 5.1 soundtrack.

[kiran_mk2]

The original tracks are there for the purists and for the fact that they will have been specifically mixed for 2 speaker setups, whereas downmixing the 5.1 track will be reliant on the chip in the DVD player...