Final answer:
The 20-mule wagon trains in Death Valley were used to haul borax, which was a significant industrial and household resource during that time.
Step-by-step explanation:
The 20-mule wagon trains in Death Valley were historically used to haul borax. This mineral was a key resource during the late 19th and early 20th centuries and was used for various industrial and household purposes. The journey to transport borax out of the valley was arduous due to the extreme desert conditions, which led to the use of large teams of mules to pull the heavy wagons loaded with borax.