The original reasons for circumcision are unknown, but several cultures have practiced genital mutilation of both boys and girls throughout history. For the Jews, they borrowed the custom from Egypt when they adapted to that culture, as Egyptians had practiced it as a means of purification. Moses himself was not circumcised, but promoted the practice as a tribal sign or a means of keeping Jewish cultural autonomy; Over time, any male not circumcised would be banished from the tribe. The practice also insured Jewish fertility, as it was forbidden for a Jewish woman to engage in sex with an uncircumcised man. See more at the link:

Actually, the practice of circumcision predates Moses by hundreds of years. If was given to Abraham as a special sign of a covenant between God and the Jews. In Genesis 17:7-10, God says to Abraham, " I will establish My covenant[agreement] between Me and you and your descendants after you throughout their generations . . . This is My covenant, which you shall keep, between me and you and your descendants after you: every male among you shall be circumcised. Thus, Moses was probably circumcised on the eighth day of his life which was the custom of the time. In fact, when Moses was in exile in the land of Midian, he did not circumcise his son and God almost killed his son because of it. The story is told in Exodus 4:23-36. Moses' wife Zipporah finally took a flint knife and circumcised their son "So that He [God] left him alone. Even today, almost all Jewish boys are circumcised on the eighth day after they are born.