Most pockets are optional, but some are incorporated into the pattern. A side seam pocket on jeans or pants is actually part of the pattern and would leave a hole, if they were not incorporated.
The image above shows a side seam pocket with a green fabric applied. The pants fabric has opacity turned down so you can see through it. Note how the top corner of the pocket is actually part of the pants pattern.
Pockets can be nearly invisible, showing only the opening. These would be like welted pockets on the back of men’s slacks. All you can see is the narrow welting around the pocket hole.
Pockets applied to the outside of a garment are called patch pockets. They are almost always applied with top-stitching. They are as decorative as they are functional.
Different types of pockets define the style of the garment. The patch pockets on the back of jeans include stitching that defines not only the pants, but often the manufacturer as well. Pockets are always an option that you can add to shirts, pants, skirts and outerwear.