save-restriction, save-excursion

[pokerface]

I am beginning to study Emacs Lisp and I have the following question:

Why is it wrong to write (save-restriction
                           (save-excursion
                               ....))

and it is recommended instead to write
                         (save-excursion
                           (save-restriction
                               ....))

Can someone please give an example of the difference?

Thank you.