Here you can see the trade-off of Focal Length (FL) and Field of View (FOV). Zooming in provides a much tighter view of the image but less area is visible. A varifocal camera provides optical zoom capability to retain the sharpness of the image without it "pixelating out".
Varifocal cameras are more expensive than fixed because of this variable focal length capability within the camera. These are best when you need to see sharp contrast at longer distances - such as reading license plates farther away.