I've never put a ID onto the BODY tag, but here's a situation where it's useful.

Normally, an article has an H1 tag that contains the title for the page.  You do this for SEO reasons, as well as semantics.

On another page, there are a list of articles.  The main headline, usually the section or category name, is in the H1 tag.  The articles within the category have their titles in H2 tags.

The problem is, when a headline is in H1, it's larger than when it's in H2.  For whatever reasons, it might be nice to have headlines that have linebreaks, so that words don't break in unusual ways.  So, on the articles page, you want the H1 tag to look like an H2 tag.

One solution is to use different stylesheets for the different layouts.  Another solution is to add an ID attribute to the BODY tag, and the ID would indicate the layout name.  Then your CSS could look like this:

#layout1 H1, #layout2 H2 { ... }
#layout1 H2, #layout2 H3 { ... }