Ash catchers are attachments for your water pipe that help to keep your water pipe clean and in some cases, they even add filtration to give a cooler and smoother hit. They trap most of the ash or junk that would usually end up in your water pipe helping to keep your water pipe cleaner. One benefit is that you will spend less time cleaning your piece. Its easier to clean a small ash catcher than most water pipes. Ash catchers come in a variety of styles. Most contain percolators for added diffusion. You can use an ash catcher for dabbing as a second filtration method. Many times the ash catcher sits as tall as the original bowl so a drop down works well when paired with an ash catcher.
Some ash catchers paired with a j-hook adapter make a great dab rig!