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.