@ryanbutlerJul 25.2007 — #I sometimes do it so that I know what specific element I'm styling. If I have multiple DIV's in a layout, its helpful to know which specific element I'm styling. Plus, it does add the greatest level of specificity you can have, because if there's a potential conflict than nothing will supersede it.
@felgallJul 25.2007 — #With classes it specifies which particular element type to apply the style to as the same class can be applied to many different elements and they don't all need to be the same type.
For an id it is effectively a commment to remind the writer what type of element they attached the id to with the added advantage that, since the style will only be applied at all if the correct element type is specified, you know that the comment must be correct.
@ray326Jul 26.2007 — #Also the level of specificity. div#id is more specific than #id and that can determine whether an element is selected or whether the style of another selector will override it. I believe Eric Meyer has an article about that on his site but it might be Cederholm.