Turn your camera into an incident meter, which allows you to measure the light falling on the subject in a fast and simple way
The Expodisc cancels out many problems found when using a grey card, namely carrying around an unwieldy item, reflection on the card that may occur which will affect you white balance reading, improper angling and difficulty experienced when filling the frame to get the best white balance setting
This great item is handy to use indoors or in weird lighting where varied light is travelling from different directions.