Cyclocross is known for it's consistency to provide the racer with varying conditions. In turn, the racer needs to have a wide range of skills in their back pocket to handle the different courses, conditions and competition.
One of such CX skills needed is the ability to have both and "pit bike" and a "pit person". In extremely muddy races the ability to change bikes throughout the race is extremely necessary. The more mud that cakes on your bike will make it increasingly harder and harder to ride. The bike gets heavier, the tires get caked with mud and don't provide traction, and the drivetrain can get jammed and stop working or break.
However, if the racer is able to get a clean bike every few laps, they will be at a huge advantage. I clean bike can make the conditions seem not so bad, allowing the racer to go faster.
But there's a catch; in order to get a clean bike every few laps, the dirty bike must get cleaned. That's where the pit person comes in. The pit person is just as busy as the racer. He/she has less than 5 minutes to get the old bike, clean the old bike and hand it off to the racer before starting the process all over again with the original bike.
The bike change should be practiced and perfected before being used in an important race because if it is muddy and you have both a pit bike and a pit person, you should use them! It will help you.