Bongs allow you to get a huge hit due to the chamber they have, even though you can theoretically get more THC from a glass pipe.
Inhaling a bigger hit “straight to the head” is really one of the easiest way to get an instant high. Bongs will get you high after just 1-2 hits while a joint will take a while to kick in.
Smoking cannabis with no additives (for instance, join packed with both tobacco and pot) will also get you there a bit quicker.
So, as you can see, there are quite a few things that give bongs an edge over other styles of cannabis consumption.
Water from the bong will usually turn brown over time, thus helping you smoke a bit healthier. Cannabis smoke is harmful (just like any other smoke really) as it contains ash, tar and several other harmful compounds which get partly filtered by the water.