Those people who favor allowing or increasing immigration levels state that newly arrived immigrants aid the economy by taking jobs that otherwise wouldn't be taken.
Arguments that are used to economically support the arrival of inmigrants
Increase in the country's total output. Inmigrants come to a host country looking for better job and life opportunities than the ones they had back in their home countries. They are willing to accept many jobs that domestic citizens reject, because they arrive in great need conditions. Their work generates a quantity of output that was not produced before.
Inmigrants are usually young individuals alone or come together with their families and they contribute to the support of the pension system in the host country. New workers are added to the labor force in a context of an ageeing population (that is the case in most developed countries that atract immigrants for better job opportunities), where the amount of people who are retired or about to retire is much larger that the size of the working-age generations. Therefore an small portion of the population has to contribute with their taxes as workers to the payment of the pensions of a large proportion of the population. Although the gap is large, this imbalance is slightly corrected with the new arrivals of inmigrants.
