MultiRes Modifier
Multires is similar to optimize in that it can progressivly reduce the detail of a mesh by reducing the vertice count. It is very useful for highly complex meshes that may be degrading system performance, stopping you from working effectivly on other objects in your scene.
Adjusting either Vert Percent or Vert Count will control the number of vertices your mesh object has, either reducing or restoring detail to your mesh.
The above example teapot uses a mesh smooth and multires modifier to demonstrate the effect.
Another use is for controling the quality of duplicate objects that may appear in the distance of your scene, and so do not require a high level of detail, thus improving overal performance.
Vert Percent could be tied to a camera to dynamically adjust the detail of objects based on their distance from it. This saves you time on adjusting every duplicate object in the scene as you move through your animation.