The smoke in a weed bong is filtered by water before inhaled. This allows you to take much bigger hits because the smoke is much cooler and less harsh. Some weed bongs have an ice catcher so you can add ice to the water and make the smoke even 20 times cooler. By taking bigger hits, more THC is inhaled (THC is the substance that gets you high). The smoke inhaled from weed bong causes less harm due to the same reason (smoke is water-filtered). Weed bong can be great if you are out of rolling papers or you just can’t roll a joint.