Since a long time, immigration is a source of economy and dynamism for most of countries. Contributing at first in great part to workforce, immigration is also a source of skilled labor permitting innovation, the holy grail of our society.
Immigration is first of all a source of cheap workforce. In the fifties, after world war 2, France needed lots of workers, so many immigrants from north Africa came in France. That permitted to the french industry to be reborn and to be competitive. We've got the same thing with the United States of America and the mexican workers. This is especially thanks to the immigration that countries like the USA and Germany can compete with China and India.
Most recently, immigration became a source of skill larbor. Many engineers and scientists are trained in China and India, but The USA still have got the largest means which allow them to attract those engineers and scientists. One of the best example is the inventor of the USB : an American formed and native from India. We know that to stay at the top of innovation, the USA need brains from all the world.
To conclude, we have seen that immigration can have a great role on economy. However that is not the only advantage, it can also contribute to a good population growth which is a crucial problem in the richest countries.