Improving the overall load time of a web page is one thing but a big factor in the user's perceived speed of a site is is how long they have to wait before something even starts to appear. How long are they left staring at a blank white space before your pretty site design even begins to render?
So it's useful to know that the <body> of the page will not appear until everything in the <head> has been loaded!
With this in mind take a look at this grab of the Firebug Net panel for the developerWorks site:
The bits in red I've added. Stuff above the red line forms the head. Below it is the body. Obviously.
Notice anything different about the grey bars of the time-line above and below the line? Above the line the files load in serial, whereas below the files (mainly images) load in parallel.
It follows that the less code you load in the head the faster the page will appear. If you can, it's an idea to limit files loaded to only those needed and to get rid of any redundant code. However, assuming you can't and that all the code is needed there's still performance gains to be had.