One of the problems with dSLR video is that the built in microphones are not that great. In fact, I would go so far to say that the quality of audio coming through them, are unusable for most situations. Also, when you handle the camera, or even focus, some of this is recorded through the camera’s microphone.
The solution is to use an external microphone, and in the case of the Rode Microphone, you can plug it straight into most dSLR video cameras. The Rode is better suited for recording audio of many individuals instead of one person. A wireless lavaliere microphone will give better quality, but every person has to have one, and if you have many people, it is both difficult and expensive to feed these all onto one track.
For most applications, the Rode is a very good value. We are using it more and more.