The purpose of the water box on a drag strip is to get your tyres wet. Getting your tyres wet before doing a burnout helps with the following things.
Less Stress: The water box helps to reduce the risk of a burnout, reducing the friction of the tyres and placing less stress on the powertrain.
Removes Dirt: It also helps to remove any dirt and debris from the tyres prior to doing a burnout.
Entry Into the Water Box
How you enter the water box is more important than you think. If your car has thin front wheels then you can go straight through the water box. However, if you have regular width front tyres you should go around and reverse into it only with the back wheels.
Driving straight through the water box gets your front tyres wet, leaving a trail of water behind the front tyres. This is not ideal as it'll reduce the grip of the rear tyres by reducing temperature and leaving a layer of water on the track.
Reverse far enough into the water box to get your rear tyres wet, but avoid getting water on your front tyres.
Don't Burnout in the Box
Burning out in the water box splashes water up in your fender wells and while your stationary on the starting line the water will drip onto the track and onto the tyres.
People may also enter the box and spin the tyres to get them wet before pulling forward to conduct the burnout. This also sprays water, so it's best to avoid it.
Doing the Burnout
Doing a burnout on a drag car with drag radials or slick tyres has several benefits.
Heats Up Tyres: Heating up the tyres can increase the grip of the tyres, but only significantly if your car has drag radials, semi-slick, or slick tyres.
Cleans Tyres: The burnout removes any debris or dirt on the tyres, such as stones, dust, etc. This dirt and debris can reduce the grip of the tyres.
Heats Up Powertrain: A burnout will help warm up the engine, gearbox, and other powertrain components.
How to Do a Burnout
Put the car into first gear, hold the brake, and increase throttle to get the wheels spinning.
Once the wheels are spinning, put the car into second gear and let your foot off the brake so the vehicle starts to move forward.
Let the car move forward for a little while then let off the throttle.