This will give you a nice crisp white border around the painting. You still have brown paper tape stuck to all the edges of you paper. Take scissors and cut between those guide lines you put down or where you put the masking tape and the edge of the paper tape and you will have a nice clean edge to your painting. If you use plywood to stretch your paper on, remember that plywood is acidic , so just don't leave your paper attached to the wood board for months. Do your painting and remove it from the board as soon as you feel you are done.
Some artists have several boards handy and stretch several pieces of paper at the same time.
One of the questions I've heard asked about watercolor paper over the years is whether there is a front and backside to the paper and how can I tell which side is which and does it matter what side I use. Hold your sheet of watercolor paper up to the light and look for the embossed watermark of the manufacturer. On cold pressed paper, the front is slightly smoother than the back and some artists swear by the front saying the paint flows and reflects better there. I personally don't see much difference between either side and sometimes use the back when I want a slightly rougher surface than you get on the front without switching to a Rough paper. It is used mostly in situations where you want to paint a dark area without having to paint around a particular spot you want to keep white.