To provide variable support
In most cases, the use of VAR is unnecessary as the nodes all have the ability to use the $-notation for variablized attributes. For example, the filename for IMAGE or the text for TEXTBOX can be specified by the appropriate attribute.
However, the node is not provided solely for backwards compatibility. There are some situations where the attribute $-notation is inadequate and a VAR node is required. (q.v. TEXTBOX for an example)
Rob Kinyon (email@example.com)