Filter cartridges for glassbongs, like precoolers, screen harmful substances from smoke, thus making it smooth in taste and leading to a reduction of pollution of the lungs. A filter is usually placed between downtube and adapter chillum. However, if you have a bong with integrated chillum, you can also place it directly between the bong and the glass bowl.
There are not only different forms of filter cartridges, but also various ways to filter. There are insets that are only ash catchers, removing larger particles wusing sieves. Special cartridges like those for the Mahony Reperc bong are similar to precoolers and have to be used with water. The most common variant are filter cartridges that use activated charcoal, to remove ash articles and reduce fine dust pollution.