With narrative song, i.e., ballads I don't see a reason, and I don't think I do it, but occasionally with a non-narrative song that is mostly chorus where there are lots of musicians and singers joining in I will do it to let them all know we're coming to the end, e.g., The Candlelight Fisherman.

Similar situation to using the commonplace:
'There's buns in the oven and cheese on the shelf,
if you want any more you can sing it yourself.'

These simple devices have a long tradition.

As several posters have said, it's a way of milking a good chorus song if it's going well and you want the occasion to go on a little more.