Why does the Fix exist?
In the late 19th century, the Bank of England needed the brokers it used to find a price for huge shipments of gold coming daily from Australia and South Africa. Today the Fixing members undertake to accept orders in any size, all dealt at that single market-wide price.
This is a valuable alternative to the 'spot' market, where the price of large bullion bars is agreed only in a unique deal between one buyer and one seller at a time. The huge volume of business done at the Fix also adds great stability to a wider market increasingly led by derivatives trading in futures and options.