The Composite Node can be used to change pictures into grayscale with a more sophosticated method than only using the RGB to BW node.
Photoshop offers a method using the Kanalberechnungen (sorry- I dont know the english translation for this function). Basicaly only one RGB chanel in combination with another is used to calculate a grayscale picture. This method has many advantages, because you are able to controll the calculation prozess very exactly, depending on the colors of the picture. So, with the Composite Nodes its no problem to imitate this Photoshop function. Actually I have no idea about its practical use, but it may be good to know, that it`s possible. In my example the blue chanel is calculated with itself, but every other combination is also possible.
I’ve been trying to think of a practical use for that node…thanks! I will add to wiki.
A practical example. Changing a sky picture to grayscale never works fine, because the difference in brightness between clouds and sky is not very big. As you can see in my examples the sophisticated method turns out the difference realy nice.
Attachments
I found better examples. Have a look at RGB to BW
http://de.wikibooks.org/wiki/Blender_Dokumentation:_Composite_Nodes_Convertor
danke. nein spreken ze deutch, unfortunately. but I may steal your pictures for the english wiki. 
I think you will find the right words in english to explain the pictures, they are are kind of self explaining…
By the way, thank you very much to stay patient, while I´m trying to write in english.
LOL. You’re english is MUCH BETTER than my german!!! Don’t apologize to me. I am not even close to bi-lingual written.