It's not to help me identify who's in a picture, but rather to make it simpler to find pictures of a person.

Example: I want to photoshop a picture of my friend onto a dog; I could either load all pictures with him tagged in it, or laboriously go through all of my pictures from a given weekend, hoping that the picture I'm looking for is there.

Result: http://i.imgur.com/C6mrd.jpg