Simply put, a percolator performs a process that is important in cannabis consumption. They use water in filtering mixtures and compounds. Quite interestingly, they have been around for quite a long time already. History has it that they were used by coffee makers of the old to filter boiling water with coffee grounds, with gravity grabbing most strength and flavor.
A percolator in a bong works similarly with a twist. The bong is consist of several sub-chambers. When filled with water, the differential pressure in the top and bottom outlet pushes the smoke to move right through the water. This process of filtration offers a cleaner and smoother hit on the lungs and throat.
You may notice that certain percolator models have smaller holes located on the bottom, called diffusers. Their role is to create water bubbles that cool the smoke and the pipe down. Overall, percolators in bongs can be really helpful for those who feel that bong rips are challenging to take in.
This is a bonus offered by the extra water contact. Better cooling helps in creating that smoother draw that results in less coughing and irritation to the throat.