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(emacs-lisp-intro.info)Narrowing advantages

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The Advantages of Narrowing
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   With narrowing, the rest of a buffer is made invisible, as if it
weren't there.  This is an advantage if, for example, you want to
replace a word in one part of a buffer but not in another: you narrow
to the part you want and the replacement is carried out only in that
section, not in the rest of the buffer.  Searches will only work within
a narrowed region, not outside of one, so if you are fixing a part of a
document, you can keep yourself from accidentally finding parts you do
not need to fix by narrowing just to the region you want.  (The key
binding for `narrow-to-region' is `C-x n n'.)

   However, narrowing does make the rest of the buffer invisible, which
can scare people who inadvertently invoke narrowing and think they have
deleted a part of their file.  Moreover, the `undo' command (which is
usually bound to `C-x u') does not turn off narrowing (nor should it),
so people can become quite desperate if they do not know that they can
return the rest of a buffer to visibility with the `widen' command.
(The key binding for `widen' is `C-x n w'.)

   Narrowing is just as useful to the Lisp interpreter as to a human.
Often, an Emacs Lisp function is designed to work on just part of a
buffer; or conversely, an Emacs Lisp function needs to work on all of a
buffer that has been narrowed.  The `what-line' function, for example,
removes the narrowing from a buffer, if it has any narrowing and when
it has finished its job, restores the narrowing to what it was.  On the
other hand, the `count-lines' function, which is called by `what-line',
uses narrowing to restrict itself to just that portion of the buffer in
which it is interested and then restores the previous situation.