Circumcision has been practiced by Jews since Abraham circumcised himself and his sons 4000 years ago. The Torah recounts the story of Abraham circumcising his son Isaac on the eighth day at God’s command, and the importance of this as a sign of the covenant of the Jewish people:
G-d further said to Abraham, “As for you, you and your offspring to come throughout the ages shall keep My covenant. Such shall be the covenant between Me and you and your offspring to follow which you shall keep: every male among you shall be circumcised. You shall circumcise the flesh of your foreskin, and that shall be the sign of the covenant between Me and you. And throughout the generations, every male among you shall be circumcised at the age of eight days.” (Genesis 17:9-12)
Brit Milah has been a central commandment ever since, through times when it was unpopular and times when it was dangerous. We are fortunate to live in a time and place where we are free to perform this ritual.
There are some medical benefits from circumcision such as decreased rates of urinary tract infections in baby boys and decreased risk of sexually transmitted disease and cancer of the penis in men. However, Jews circumcise their sons because of the commandment to do so, not because of medical indications.