Parallel markup: Why do the links point from content to presentation?

Dear all,

  could you probably tell me a use case that justifies the direction of the
links in parallel markup?  They point from the content markup to the
presentation markup.  OK, it's easy to obtain the reverse direction as well,
but there must be some background behind the decision to have them point that
way.  For the presentation->content direction, I can imagine lots of obvious
use cases, e.g. selecting a rendered term and then issuing a command like
"compute this", "prove this", "rewrite this".  For the content->presentation
direction, is the use case something like: in a CAS, you look at a
content-oriented source code, and when you select part of it, you get a
presentational preview of the selection?  (Maybe it would make sense to
mention some of these use cases in the spec.)

Thanks in advance,

Christoph

-- 
Christoph Lange, Jacobs Univ. Bremen, http://kwarc.info/clange, Skype duke4701