Why do bongs have a carb?

A carb hole is the most often used while smoking out of the glass bong that is not equipped with a pull out glass on glass bowl and slider. Therefore, instead of pulling out the slide you firstly cover the carb hole with your finger and once the chamber is full of smoke you take the finger off and inhale. Carb holes are the most often placed on the back side of the bong or on the left side.

Carb hole bongs are usually cheaper than the glass on glass bongs. The reason is that making glass on glass bongs is more laborious and also the bongs with a pull out slide and without a carb give stronger hits. However water bongs made with a carb hole are still very popular.

Water bongs with a carb hole and a pull out slide

Some water bongs can have both - a pull out slide and a carb hole. In this case it's only up to you, which way you want to smoke. Smoke-nut.com sells these types of bongs as cheap bongs.