First advantage that comes to my mind is transparency. If you compare a blockchain auction to a 'regular' auction you can see all bids in the blockchain but in a regular auction you can't be sure whether the auction keeper has for example ignored some bids (unless all the bids are done physically and visibly).
Another advantage (and to some degree, disadvantage) is the ability to not censorship the contents. So everyone see all bids and evreyone sees whether the auction contract is able to block some bids automatically, for example. This is closely related to the previous point.
Third would be availability. Anyone around the globe can participate without long registration processes and the auction can't suffer from various outages (power failure, problems accessing the physical location, etc).