Firstly how immigrants and their children will success in labor markets influences directly how much they pay taxes and how much they receive income transfers.
Secondly, most of immigrants are coming to Finland in their best working age, therefore their early years have been funded somewhere else. Immigrants usually move out more often from Finland, therefore they reducing the public service costs from the rest of life.
Immigration creates advantages for the production of collective public services. Some of publicly produced services, such as defense spending, can be maintained roughly the same cost regardless of the size of the population. Increase of population