Welding isn't just to join two or more parts together. it can be used to finish off a part, or join it to itself. You can add a backing weld, or a surface weld to just a part to strengthen, beef up or add materiall to later remove. Sometimes you'll add welding as a wear surface, like on the edge of a shovel, a plow, or a mixer blade..
In sheetmetal, you will create a box, and the folded up edges that make the box you want welded together (seam weld or fillet weld) to make a sealed 'pan' or box with nice, neat edges. A pipe usually is a rolled flat sheet into a tube, then the seam is welded..
