I have tagged other people when iw ant to be sure they see the picture, and when I'm pretty sure a large number of their Facebook friends will want to see it.
Like when my friend's kid is over (we're babysitting), and he does something cute, and we take a picture. I'll put it up and tag her. (She puts her own kid on Facebook, so I'm sure she doesn't mind.) That way his grandparents, aunts, godparents, family friends, etc., get to see him being cute as well.