Typing document.getElementById('nodename') every time you need to address a specific node is a lot of overkill.
Take for example this block of code:
With the coming of the prototype javascript library, scriptkiddies all over the world just use the $ sign to address a node, if they had first defined the following function, also present in that library:
Note that the function name is in fact only "$", a sign used in other programming languages, not javascript, to denote a variable.
Once defined, the previously stated block of code can be rewritten as:
Taking up less characters...