I’ve finally learned that the Width tool is used to adjust the thickness of the line stroke. You can play with it to see how it works, then if you’re happy with the stroke you can leave it as you like. I also learned the Profile is what functions the different shapes and styles of the stroke when the Width tool is used. It’s at the very bottom of the Stroke panel. It’s shown a straight line with “uniform” on the right at first, but when I select different styles, it changes the shape of the stroke. The Profile is also located at the top of the screen in the Width panel. You may add different types of Brushes (next to Profile) if you wish.
Alice's Blog 