1 - To achieve a flat, tight surface and prevent the paper from buckling during the painting process, which causes the washes to puddle in the valleys.
2 - To break up the sizing on the surface of the paper. This sizing inhibits the initial washes from locking in the paper fibers.
There are several ways to stretch your paper, but I will describe and show the staple method, which I consider the fastest, easiest, and most effective. With this method, I use 140-lb, cold press watercolor paper, soaked and then stapled onto Gatorboard.