The Lens plugin allows you to add or remove lens distortions from your rendered imagery. This is especially important when compositing your LightWave imagery onto an existing background. If that background was filmed with a wide-angle lens, you simply cannot accurately composite your LightWave scene. LightWave's camera models a distortion free camera which cannot match the real camera's behavior.
The Lens plugin allows you to add many different camera distortions to your rendered imagery to match your background plates. These distortions include shear, offset, scaling, rotation and pincushion effects. You can also apply the inverse distortion to an image. This lets you take a background plate and remove the distortion from it to get a "flat" version. This flexibility allows Lens to distort your rendered imagery to match the background, or to undistort your background to match your rendered imagery.
Lens is not very important to animators who do not use compositing, but essential to those who do.
Lens can also be used for special effects like automatically shrinking and moving your imagery to a corner of the screen (for over-the-shoulder TV news graphics), rolling the picture (like on a display monitor with poor vertical sync), shaking the image, or other goofy image manipulations.