Video is not all about the images. The sound is at least as important, and needs just as much effort to get right. The camera's in-built microphone will only record the sound where the camera is, not where your subject is! This is why DSLRs (and many compact system cameras) have external microphone sockets. You can plug in directional 'shotgun' mics or lapel mics for interviews or subjects talking to the camera – you can also use wireless radio mics.
Sound travels more slowly than light, and the further away your subject is, the more the sound will be delayed. If you record the sound from the camera position it will get out of sync. Not only that, you're more likely to pick up sounds made by the operator or the camera.
Many videographers record sound separately using an external sound recorder and then synchronise the sound with the video later – especially where the camera's internal sound recording system doesn't provide the desired quality.
