g/pat/s/pat.*/to/ vs :%s/pat.*/to/

[Yakov Lerner]

In vim docs, there is example like this:
      g/pat/s/pat.*/to/
As far as I know, it is also possible to do it without g:
      %s/pat.*/to/
Is there a difference, a reason to prefer the
first form, the longer for with 'g' ?
Thanks
Yakov