Pose,

You put it where your subject is, point its globe where the camera will be, and it will tell you the exposure setting for that particular shot.

It won't be fooled by white or black or strong highlights or reflections. You can also meter an area and set you camera to thos esttings and walk around shooting knowing that they exposure will be prety much perfect unless the lighting changes. THey also allow you to meter studio flashes, they are pretty much necessary if you want to get anything done in a reasonable amount of time.