Vari-focal lenses allow you to adjust the field of view so that the area you want to monitor fills the screen. When you are narrowing the field of view to concentrate on just an area of interest this ensures the maximum amount of detail in the recorded image.
This is what professional installers would fit in order to maximise the likelihood of gaining evidential recordings.
It is the 12mm / 24Â° end of this lens' adjustment range which will probably be most useful to you. 24Â° is a fairly narrow (zoomed-in) field of view, it may be ideal for example for covering only the entrance doorway into a shop from a camera mounted at the rear of the shop. By adjusting the camera to show only such a narrow field of view, you are very likely to achieve good evidential video.
This lens allows you to adjust the view to in excess of 90Â°. Whilst this can be ideal for applications where you wish to mount the camera in the corner of a room and cover the whole room, please bear in mind that as the scene is adjusted to be wider it also inevitably becomes taller - this means, wide scene = small people in the scene = less detail about those people!!
Also, please bear in mind that as you adjust the field of view to be greater than 90Â° you will begin to see a fish-eye effect; the walls, ceiling, and all straight edges will begin to curve.
This lens, at a wide setting, may be just what you need for 'monitoring' a large area, but it is unlikely to be appropriate if you are trying to achieve evidential recording.