As far as fixed width layouts are concerned, there are many reason why you would want to do this. A few include:
1) Fixed width is easier to design for.
2) Variable width layouts can become difficult to read on exceptionally large (or small) resolutions.
3) Browsers handle the box model differently. Sort of goes along with point number 1.
Ultimately, both have their merits and demerits. I try to target an average resolution while making the page usable in higher and lower resolutions.
Mark