Board Thread:General Discussion/@comment-97.95.138.90-20140330004141/@comment-10390252-20140404064229

FWIW, when Aura was revealed, it was speculated (this has never been confirmed in canon) that Aura acts as a 'trigger' energy to set off a Dust reaction.

Whilst Dust can be used without Aura (so it is an energy source for machines too), Aura boosts and is boosted by the interaction with Dust. So Dust is also a catalyst for bio-etheric energy (woo! Final Fantasy - The Spirits Within reference!) that increases the scale and utility of its effects beyond the narrow applications available through Semblance.