The very small number of scripts that belong to the first of these four groups make up the entire group of scripts where there is a benefit to attaching the script to the head of your web page. Attaching it there means that the code can be loaded and run as soon as possible. Such scripts generally redirect some visitors so that the current page gets replaced by another page (such as a framebreaker script that replaces the current page by the content of the current frame without the other frames). Ideally this will happen soon enough that your visitor will not even realise that the redirect has taken place.
For all three of the other groups (two of which contain almost all of the scripts you would ever use) placing the script tag at the bottom of the body
The first consideration in placing the script tag at the bottom of the body whenever possible relates to how the HTTP protocol works for browsers to load the files (CSS, images etc) associated with the web page. With the exception of one type of file, early browsers could load up to two files at a time per domain. More modern browsers can now generally load up to eight files at a time with that same exception. By loading files simultaneously the files can be loaded much faster than if they were loaded one after the other because the limiting factor on how quickly a file can be delivered to your browser is not the capacity of your internet connection and so downloading multiple files will better utilise that capacity and get the files loaded faster. The same web page will finish loading a lot quicker on the same internet connection in a browser that allows eight files to download simultaneously than it will in a browser that only allows two files to download simultaneously.
This article written by Stephen Chapman, Felgall Pty Ltd.