What is the difference between a Viewer and a Compositor node? I was messing around with a scene earlier today and looked up some tutorials on chromatic aberration, and I noticed that all the tutorials I found had both a compositor node and a viewer node?

Is there any real important reason for using one over the other? Both export to the UV/Image Editor and allow saving out to .png files, and both seem to create an identical final image with my depth of field, glow, and chromatic aberration. Are there situations where one is better than another?
As per the manual
They are two of several output nodes and allow you see or save the working image in progress. At any point only one viewer and one composite node is active.

The Viewer node is a temporary, in-process viewer. Plug it in wherever
you would like to see an image or value-map in your node-tree.

and

The Composite node is where the actual output from the compositor is
connected to the renderer. Connecting a node to the Composite node
will output the result of that node's full tree to the Renderer,
leaving this node unconnected will result in a blank image. This node
is updated after each render, but also if you change things in your
node-tree (provided at least one finished input node is connected).