Summarize:

Stitches have a multitude of uses. The most common reasons to get stitches are:  To close a wound that is too big to be closed otherwise. Using stitches to bring the edges of the wound together can help to speed up healing. To prevent infection. If you have a large, gaping wound, closing it with stitches can help to minimize the risk of infection (as skin that is broken open, especially large, gaping wounds, are a prime target for an infection to enter the body). To prevent or reduce scarring after your wound heals. This is especially important when the cut is on areas of the body that are more cosmetically important, such as the face. If it is greater than 1/4 inch deep, the wound may be eligible for stitching. If it's deep enough that you can see yellow fatty tissue, or even bone, you should definitely see a doctor for treatment. Are the edges of the wound close together, or do they need to be pulled together to cover the exposed tissue? If the edges of the wound need to be pulled together to cover a gap of exposed tissue, this is an indication that stitches may be required. By pulling the edges of the wound close enough to where they can touch, stitches can help to speed healing. If the open wound is located on a specific area of the body where there is a lot of movement involved, it will most likely need stitching to prevent re-opening of the wound caused by movement and stretching of the skin. For example, an open wound on the knee joint or fingers (especially where joints connect) would be eligible for stitches whereas an open wound on the thigh would not really need stitching. Tetanus shots last no longer than 10 years and then you'll have to be re-vaccinated. If you have an open wound and it's been longer than 10 years since you've had a tetanus shot, go to the hospital. While you are at the hospital, you can have the doctor evaluate the cut also to see if it will need stitching.
Understand why stitches are used. Consider the depth of the wound. Evaluate the width of the wound. Look at the location of the wound. Ask your doctor about getting a tetanus shot.