Imagine you have a situation where you want to append Body content to a number of Drupal pages, according to content type.
e.g., to all "Stories" content add "Please contact Mr. Blah on 0400 123 123" at the end of the Body.
You don't want this as block either - it's the sort of thing that (for example) must be printed. It has to be appended in the 'Body' section of the node.
If this is a couple of pages perhaps you could just copy and paste. But for larger numbers this is not recommended. Imagine if you had to modify twenty nodes, or two thousand.