Basically, it avoids re-inventing the wheel.
- Faster time to market
- Less Cost
A good example:
A bike rider notices his hands are cold. Being a smart person he decides to come up with a solution to keep his hands warm while riding his bike. He spends lots of time and effort coming up with his "hand warming system" which does keep his hands warm.
Confident, he shows up to the next ride ready to show all his friends his new "hand warming system". When he arrives, he notices his friends all have something on thier hands. When he asks he friends what they are and they respond: "They are gloves, for keeping your hands warm."
So, even though his "hand warming system" worked, he wasted a lot time and effort because there was already a proven solution available.