DESTDIR direction, definition?

["Justin C. Sherrill" <justin%shiningsilence.com@localhost>]

Is there a description somewhere of what DESTDIR support does and what its
benefits are?  I've been mentioning it when dealing with pkgsrc material,
and I can infer that it allows building packages without being root.

Reading from other descriptions, it looks like it's part of a plan to have
pkgsrc be able to build binary packages and then install from them safely,
in an effort to remove the major problem of packages not rebuilding during
an upgrade and leaving your systems in disarray.  Or maybe I'm pushing my
own hopes on it.

Can someone elaborate?  I've been using the term on the DragonFly BSD
Digest without having a proper definition or target to explain it, and I'm
sure it's confusing all 3 of my readers.