Psalm 23 — what do the rod and staff actually do?

The rod defended the flock (against predators), the staff guided and rescued individual sheep — the crook. So the comfort is: protected from outside, guided from within. It's covered beautifully in week 1 of the Psalms course.

"Thy rod and thy staff they comfort me" — I've sung it a hundred times. What did a shepherd actually use these for?