What is the purpose and basis of the stigmata?

The stigmata is sores or wounds in the places that Jesus suffered wounds during the crucifixion. That's about as much as I know. My questions, to be answered from a Catholic perspective: 1. What's the purpose of the stigmata? 2. What is the Biblical basis, if any, for the stigmata? 3. How does the stigmata not indicate Christ's suffering was insufficient? 4. How do we know that the stigmata reflects Christ's crucifixion and not someone else that was crucified?