Bundles explained

I'm having some difficulty wrapping my head around when bundles would be
used.  Is it so we can describe how a set of provenance records came to be
(the provenance of the provenance)?

I'm having a little difficulty wrapping my head around the use cases.

Example 40 from
http://www.w3.org/TR/2013/REC-prov-dm-20130430/#component4shows two
reports (r1, r2) being generated with r2 derived from r1.  It
then describes a bundle describing that "Bob" witnessed r1 being
generated.  The example goes on to show a bundle for "Alice" observing the
generation of r2.

How is this useful?  I think my real question is shouldn't all provenance
events be contained in a bundle?

Any insight is appreciated.

I'm working on a clojure implementation of the provenance model as an
exercise and I want to be sure I have my understanding set.

Thanks.


--
Mike Loll