Unique ID Special Value

[Jeff_Hager]

Need Plain English Explanation of Unique identifier

I feel like I am understanding most of what glide offers (and how I would use it) except now the Unique Identifier. Can someone give me the basic explanation on what it is/does but most of all why I would use it in the most basic way? I have read the announcement and the tweet but am not real sure what it is. (I did read a couple of replies on twitter that asked the same thing.) Anyone wanna offer a 1-2 sentence WHY?