An empire with the extent of Rome could have not been successful if it had not been for the great discipline in organizing her provinces.
The power of roman economy based on farming and trade. Both two factors were interconnected with business of the provinces. Roman empire enjoyed a great free internal market around the meditation sea, where most of her provinces located. This hub of business had a center which was the great city of Rome. The overcrowded, super populated city of Rome with spectacular and luxurious way of life, was in need of economic assistance of all provinces to survive.
The most important of necessary commodities were the agriculture goods and the most important
one of them was grain, which was the main source of food for the romans. As the Rome population were increasing and state slaved sponsored farming in Italy were not answering the ever-increasing need for grain, they had to fulfill that need from the sources within provinces such as Sicily, Tunisia, and Egypt. The grain it was not the only agricultural product was imported from the province, the large amount of olive oil from Spain and wine from Egypt are among many other agriculture commodities imported from the provinces.
Rome was also the destination for food commodities such as fish, salt, meat and all kinds of fruits from her provinces. Pottery, wood, textile, tin, gold and silver were part of other commodities which were shipping in large amount from all over the empire to the great capital.
The provinces were also being a great source of slaves for slave based economy of Rome, either at a time of conquer which portion of their population had been sent to Rome as slave or in the later time that was the harbors of transporting for new slaves from all other parts of world to the Rome.
The other important effect of provinces on roman economy was the taxes. The roman state taxed provinces heavily as they were an important direct source of income for the government of Rome. The government were exploiting the provinces in any way possible and even if they were trading among themselves in their markets they would have to pay the tax of those commercial activities to the roman masters.
In general, the city of Rome was similar to the heart in the body of roman empire and without the blood pumping through the vessels of provinces, she would not have survived.