What happens if a large wound isn't stitched closed?

Wound Healing slow. Large wounds need to be properly cleaned, all dead, devitalised tissue, and contaminants removed. This is the most important thing. The wound edges need to be closed as much as is possible to reduce healing time, and improve scarring. Less of all the above will cause a worse scar, and slower wound healing.
Time will heal. Stitching a wound closed dramatically decreases the time to wound healing. But if a wound is left open and properly cared for with moist gauze dressings twice daily, the wound will contract and close on its own.
Heal slower. A large gaping wound will take longer to heal. If a doctor determines it is safe to close a wound with stitches, it will then heal faster.