The purpose of this act was to limit the number of the “wrong” kinds of immigrants who could come to the United States.
In the three or four decades before 1924, there had been a large wave of “new immigrants” coming to the United States. These people were called “new” because they were from different countries than previous immigrants had been. They were from Eastern and Southern Europe instead of from Northern and Western Europe. Many of the immigrants from Eastern Europe were Jewish. Many of the immigrants from both areas were politically radical. All of these factors made many Americans very worried about the country’s society. They felt that it was being changed by these immigrants who came to the US and did not (it was said) want to assimilate. They feared that these immigrants were from racially inferior groups. (In those days, different kinds of white people were seen as very different from one another.) They wanted therefore, to limit the immigration of such people.
This was the point of the National Origins Act of 1924. It set quotas for immigration that gave preferences to the sorts of nationalities that had been in the US prior to 1880. By doing so, it permitted more immigration from those countries and less from the Eastern and Southern European countries from which the “new immigrants” came.
The purpose of the National Origins Act of 1924 was to restrict the flow of newcomers from the Southern and Eastern Europe. What it did was that it established quotas which discriminated the newcomers. This would be the reason for the decrease on the numbers of Europeans immigrating to the US in the 1920s.