How to use `narrow'?

[Ilya Zakharevich]

My idea how *I* would use `narrow' is like this:

  Suppose I have a file which is "in multiple modes".  E.g., consider
  some "literate programming" style text, or one which contains a
  HERE-document with a program in a different language than the main
  text of the document:

    $var = <<EOV;                # E.g., this is Perl

       // Here is some code in JavaScript
       ...

    EOV

  So I would like to narrow to the region "inside the HERE-document",
  and switch the mode to "other mode" (above, from CPerl mode to
  JavaScript mode).

However, I looked, and `font-lock' does `widen' before it starts up;
so all the effect of narrowing is gone...  Is there any *use* of
narrowing?

Thanks,
Ilya

P.S. I ask this since now CPerl has a (contributed) code to
     `narrow-to-here-doc', and I would like to document why this would
     be useful...