I guess in theory you could use it as a primitive set of scales if you balanced a fulcrum exactly in the middle of it so the iPhone was balanced level on it. Then you'd need to have weights of the appropriate weight with you as scales used to be in the olden days. You put the weights on one end of the iPhone, and the item/substance you want to measure on the other end and a spirit level like app would show you when the items being weighed were equal to the weight on the other end by becoming level.
All in all rather a lot of hassle compared to actually carrying an electronic scales with you when you need to weigh something!