While big city political bosses and their machines were often criticized, they proved necessary and effective in the new urban environment because.

they were more effective in serving urban immigrants needs than weak state or local governments.

Explanation:

While the Americans' attitude towards the immigrants was intolerant, it was also reflected in the government policies that keep the immigration under strict control. The American population was in threat to the cheap labor from other countries would leave them with no jobs and therefore hostilities towards the immigrants were evident. But among these scenarios where public opinion and government were both against the integration of the immigrant's big political bosses served their needs of employment.