People from slums bring in goods from the city to sell to people living in slums. The result for the city is that the formal/ city economy cannot be evaluated as accurately because of informal shops in the slums selling those goods.
These trades in the informal economy are basically "hidden" from the formal and documented, economy. The economy cannot be valued accurately because at one point in the cycle of trade, the good disappears from the market: Costo sells 5 oranges in bulk for 15 bucks, a guy from a slum buys those oranges for 15 dollars, Costco makes a 15 dollar profit. (heres where the informal economy starts and where it becomes hidden from the market) the guy brings it to his shop and sells the each orange for 5 dollars. This guy gets a profit and someone else just bought a good. This half of the process is completely undocumented therefore it took place in the informal economy. This may happen or the exact opposite may happen. The orange seller could have grown his own oranges and sold each for 3 dollars, either way the formal, documented, economy would not know.
So, in Rio, this is a problem and/or a benifit, depending on how you look at it. This is a problem because some good and services are going being sold and provided off the market which effects the GDP of Brazil. Also, this further increases the wealth gap when products go off the market and are sold at a lower price within poorer communities: it keeps the money from the rich, with the rich, and the money from the poor, with the poor; their is no circulation. The upside to this is that, since there are so many people living below the poverty line who can't afford the cost of living in Rio, the goods and service brought off the market and sold and provided at a lower cost give the people in Favela's affordable costs of living. Unfortunately, a huge portion of people live under the poverty line and rely on these goods to that go off the formal market and into the informal economy.