Why was it important for the amry to gurad the gold mines and the trade routes?

First off, what country/nation is the question based on?

I will go ahead and give you a generalized answer that could apply for any country/nation:

In the past, it was important for the military to guard is gold reserves and mines so that other "enemies" couldn't just go walk in an steal their gold from their mines or reserves. And pretty much the same applies with trade routes, and also so people couldn't go hijacking their trade routes and steal goods from the people travelling the route.