Because buyers have to have cash in hand or finance settled 'come what may'.You are buying property with no full survey and which may not be something upon which many, or any, lenders will lend. It's quite common to find that the legal pack which comes with the property before sale contains some unhappy surprises, the significance of which is not fully apparent to a layman. The sale is made on the fall of the hammer, no messing, and you have to pay on the spot, or a deposit and the balance in a few days, usually 14. In Scotland, where you don't have the legalised avoidance and argument that the English system allows, this may not matter as much.
For a seller, the benefit is speed but the hazard is the room. Anyone familiar with auctions knows that some lots are sleepers that fetch far more than estimate, others barely make the auctioneer's discretion figure below bottom estimate, others don't sell and have to be put up again later, perhaps at lower estimate, and similar properties may vary widely in price in the same auction or in another in the same period.