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In our chemistry curriculum we are doing these lab experiments where we are supposed to test salts for certain anions,cations and other stuff.

I was told that if I dissolved the salt in concentrated $\ce{H2SO4}$ and then heated it for a while over a flame, the evolution of light brown vapours would indicate the possibility of $\ce{NO3-}$. To confirm its presence, however, I would have to repeat the test but this time also add some copper bits to the solution and check for dark brown vapours.

I figured the reason was that the copper reduced the $\ce{HNO3}$ gas to $\ce{NO2}$ which increased the amount of $\ce{NO2}$ gas produced, resulting in a darker shade of brown.

My question is that why copper, which is a very weak reducing agent, used here? Why not a real reducing agent like zinc which will give more $\ce{NO2}$ even with lesser salt?

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This is a preliminary test, which is neither particularly selective nor sensitive. With nitrate, brown vapours of $\ce{NO2}$ are released. However, bromine or iodine could also release brownish or violet vapours. Copper (or silver) is used to remove bromide and iodide by precipitation.

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