What motivated and sustained the long-distance commerce of the Silk Roads, Sea Roads and Sand Roads?

Answer question 1 or question 2.

Question 1: What motivated and sustained the long-distance commerce of the Silk Roads, Sea Roads and Sand Roads?

Question 2: How did China influence the world of the third-wave era? How was China itself transformed by its encounter with a wider world? Extra credit: What are some other open ended discussion questions that I might have asked?