It is to keep people from talking to other buyers and making deals. If you are looking for a racehorse, you won't have time to ask another bidder what is highest he/she will go on a specific horse that you also want. That way, the seller and auctioneers get more money.
1) They're usually on a time limit 2) By speaking quickly, people buying have to think more quickly, and if they don't act quickly enough, they'll lose a deal or sale 3) Usually only the bidders that will pay for more are intelligent enough to listen to the quick speech, because those with the msot money are successful, and being a good listener helps you achieve success.
They're somewhat subtle, but do have an end purpose.