What is a page-wrap div and what is it used for?

It all depends on your css layout you are applying... Sometimes, people use wrap containers to use sticky footer technique (header and content in the wrap, footer after the wrap - google it); sometimes with this wrapper the designer just wants to fix the entire page to certain width and center it (not saying that this is the best way or that there is a best way), etc... It is really subjective, because it is mostly a design thing.