Image updating is useful if you have dynamic graphs or perhaps a webcam. The main trick is appending a date string after the image file name to make sure the browser does not use a cached version of the image.
This is because you're getting a set of changes that bring your query snapshot current with the initial state of the query.
I see that apps like facebook for example are getting push notifications in real time.
When someone writes a message to me from a pc ill get the message to my android device as push notification just a second later even when my facebook app is not running.
This avoids you needing to 'poll' the db.
An initial call using the callback you provide creates a document snapshot immediately with the current contents of the single document.