Quality watercolor paper is made from cotton, and cotton shrinks. If it does that while you’re working on a painting, the paper will ripple and the paint will go into puddles and change the image. To keep that from happening, before I start I pre-shrink the paper — for a large sheet, by wetting both sides in the shower! Then I hang it up to dry, free hanging so it slowly shrinks down.
This keeps the paper from changing shape as much while painting, and makes framing much easier. I’ve been known to re-wet just the back of the paper after a painting is done and carefully iron it so it sits more nicely in the frame.