The ash catchers are also bong stems that slide into your water pipe and help create more bubbles thus more smoother hits for the user. Unlike the other bong stems, the ash catcher also retains any debries like tart and resin from getting into your bong thus the user will have a cleaner water pipe for long periods of time.
Usually the ash catchers are much more efficient than any other bong stems because they add an additional water pool to your bong. So the smoke will have to pass through two water pools where is cooled and mixed with water resulting a much more purifed, cool and crisp hit. Also an ash catcher has more slits than a regular bong stem which increase the bubbling and mixes the smoke with the water. After passing through the ash catcher's water pool the smoke will have to pass through the bong's water pool where will be furthermore cooled and purified.
You can look at this Eleven Arm Tree Ash Catcher as a much more powerful and efficient bong stem. It will add an extra water pool to your bong, the eleven diffusers and 33 slits will brake the smoke into small bubbles for the best hit you ever had and also will maintain a cleaner water pipe. No more rubbing and cleaning your bong with cheap bong cleaners.