Gauge Swatch Questions

Hi. I was wondering why some people when their done with making a swatch they pin it up and spray water on it and check the gauge again?.

Does a person have to do that?

Because some yarns stretch out with washing or shrink up or not change at all, but it’s good to know. The things I make don’t usually require one, but when I have I measured then washed and dried it then measured again. How I washed it/dried it depends on the fiber.