Either to determine your position or verify the accuracy of your position determined using some other means.
Or perhaps were you able to say "Hey, I was within a mile and a half of my actual position!"
Or did your sextant go aboard in its box and then disembark without coming out on deck.
Or did you take a sight, mark the time and then lose the piece of paper in your pocket?
I am guilty of the latter on more than one occasion.