Digital Low-Pass Filter When Oversampling Audio
However, once I've sampled at 160KHz, why would I want to apply a
  digital low-pass filter before decimation? What are the advantages of
  this?


Aliasing, as a problem is not reserved purely for the analogue world - you also need to low-pass-filter it in the digital domain when decimating. The same rules apply - you need to restrict your bandwidth whenever you sample something in order to avoid aliasing or you will get exactly the same problems of folded spectrums.