What are they good for?

Dashcams have many applications, but most importantly, they act as an impartial witness in case of an accident. Sadly, there are dishonest people in the world, and a dashcam provides irrefutable evidence of what happened before, during, and after an incident. No more "your word against theirs" - rest assured you will always have the truth on your side.  

Many dashcam models provide GPS data logging as well. These camera units, along with the video, will provide a record of your position and speed at all times. This information, again, proves the facts such as your whereabouts at a particular time, and can also be used to prove (or perhaps more importantly: disprove) your speed. This GPS data can be overlaid on a Google-type map during video playback on your home computer.