You don't have to replace that carpet, we can repair it!
Pet Urine damage: When just cleaning the surface fibers in not sufficient.
Your pet gets locked in a room and claws at carpet seams by the door.
Your cat uses your carpeted steps as a scratching post.
Permanent stains such as plant stains, red beverage stains, and burn spots.
Remodel of a room such as moving a fixed structure.
So, when you buy new carpet you always want to keep extra. For example, stairs wear out first so get enough to replace your steps in five to ten years.
You may also need extra carpet for bonded inserts. If there is no donor carpet, often times it can be taken from an inconspicuous area such as the corner of a closet.
Now, the donor carpet is obviously going to be newer and have less soil and wear conditions which could result in a contrast between the two pieces. Over time the donor piece may wear some and reflect less light and blend in better. What I mean by "reflecting less light" is a new fiber will not have any scratches, nicks, or soil on it so it reflects more light. Over time nicks, soil, and scratches make the carpet appear dull.
Remodeling, such as removing an area of carpet and installing hard surface flooring and then needing the remaining carpet stretched up to the edge of the hard surface flooring.
Poor installation: Frequently we come across carpet that needs to be re-stretched because when they were installed the installer used a flat tool called a knee kicker because it is the fastest method.
Unfortunately this tool fails to stretch the carpet as tight as needed to prohibit the carpet from loosening later. The proper method that is required is to stretch the carpet with a "power stretcher".
Once the carpet is properly installed it will never need to be re-stretched again. The only exclusion to this would be a defect in the carpet. The only way to determine if the carpet is defective is have it inspected by a carpet inspector. Carpet defects are very rare.