How is it that all prophets were Muslims, and bore witness to Muhammad's message before his advent, and what is the benefit of Judaism and Christianity if Islam is the original religion?

The fact that Prophet Muhammad (peace be upon him) came chronologically after previous prophets does not mean they were unaware of his advent. Indeed, Jesus (peace be upon him) foretold his coming, and Allah took a covenant from all prophets that they would believe in him and support him if he were sent in their time, as stated in the verse: "And [mention, O Muhammad], when Allah took a covenant from the prophets: 'If I give you a Scripture and wisdom and then there comes to you a messenger confirming what is with you, you must believe in him and support him.'"

Islam has two meanings: a specific one, which is the Sharia of Muhammad (peace be upon him), and a general one, which is submission to Allah through monotheism, obedience to Him, and disavowal of polytheism. All prophets were Muslims in the general sense, and the Qur'an clarifies that Islam is the religion of all prophets, as Allah says about Abraham: "Abraham was neither a Jew nor a Christian, but he was a monotheist, a Muslim." The prophets differed in their laws (Sharia), and the Sharia of Muhammad (peace be upon him) is the final and abrogating law.

As for Judaism and Christianity, these are names of attribution, not originally religions. Christians are attributed to Nazareth, and Jews to Judah son of Jacob. The followers of Moses and Jesus adhered to the religion of Islam to which their prophets called them, as the disciples said to Jesus: "We have believed in Allah, and testify that we are Muslims," and Moses said to his people: "If you have believed in Allah, then rely upon Him, if you should be Muslims."