The best answer would probably be trade when considering the duration of their whole empire.
However, a good portion of the starting capital Mongols had was received by looting and ransacking cities. The Mongols were actually notorious for ransacking cities and taking everything that was valuable, also killing the local population.
They did, on the other hand, also establish trade with different neighbouring nations which allowed them to gain additional streams of revenue.
They were the german police
because it is one of the best arming areas in the word yes
"It brought in more capital" is the best answer as to the effect that increased trade had upon many countries. Before this, however, many explorers were sent to find gold.