It's there, even though it's empty, because supposedly the original DVD spec called for both folders to be present. Although old DVD players might insist on the presence of an AUDIO_TS folder, I don't think any contemporary players do.

DVD audio is a format that allowed for much higher sampling rates and bit depth that regular CD audio. But since regular CD audio is considered by most engineers to be "good enough" to cover the abilities of the human ear then there has been little incentive to go the DVD audio route.

plus super audio CD is it's competitor. Differnt musicians prefre one over the other, hence no standared like DVD. Well, there is... there's two fo them. :)