The answer to your question is ... that it is of great benefit to America and to all Americans for the Federal government to secure all of our inalienable rights with which our creator has endowed us. US citizenship is the inalienable right of all persons who are born in the United States, and no State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States.
You would have a much clearer understanding if you were to properly word your question, e.g.
In what way is it beneficial to America and to Americans to have people who are born in the US be US citizens (noting that this is completely independent of parentage because the parents are other people, which is why there is no parentage qualifying clause in the 14th Amendment, and also noting that the child born in the US has not committed any crime and cannot have his inalienable rights violated over some infraction committed by others) ?
Don't be afraid to come to me with the hard stuff.