The reasons for the emergence of a monotheistic religion that supplanted (for some) earlier, polytheistic religions cannot be derived without at least some reference to the Bible, specifically the Hebrew Bible. Judaism as a monotheistic religion has its origins in the first five books of the Hebrew Bible, which comprise the Torah, and the origins of the Hebrew Bible are subject to considerable discussion. Some forms of monotheism did precede the emergence of Judaism; ancient Egypt practiced a form of monotheism known as Atenism, referring to the God Aten. Atenism, however, was not fully monotheistic, as it did not reject the notion of other gods per se but elevated the sun-disk god Aten to the place of primacy. Written histories were few and far between at the time, and the Bible is a principal source for the history of Judaism. In his Histories, Tacitus wrote of the origins of the Jews who populated the region the Romans called Palestine:
"It is said that the Jews are refugees from Crete, who settled on the confines of Libya at the time when Saturn was forcibly deposed by Jupitor...while there are many who think the Jews an Ethiopian stock, driven to migrate by their fear and dislike of King Cepheus. Another tradition makes them Assyrian refugees, who, lacking lands of their own, occupied a district of Egypt, and later took to building cities of their own and tilling the Hebrew territory and the frontier-land of Syria."
Later, Tacitus describes the beliefs of the Jews as follows:
"The Jews acknowledge one god only, of whom they have a purely spiritual conception. They think it imperious to make images of gods in human shape out of perishable materials. Their god is almighty and inimitable, without beginning and without end. They therefore set up no statues in their temples, or even in their cities, refusing this homage both to their own kings and to the Roman emperors."
Tacitus, as a more thorough reading of his writings reveals, was no admirer of the Jews. As a Roman senator, his loyalties were to the polytheistic empire that he served, and the Jews were viewed in a less than benign light. This suggests that the advantages that accrued from converting to Judaism in the period weren't always obvious; converts often faced the antipathy of adherents to other religions, like Tacitus.
The question of what advantage was to be derived from adherence to a monotheistic religion is a bit problematic. It ignores the importance of the Torah to Judaism and instead assumes a more mercenary motivation behind the development of the theological aspects of the religion. Assuming one rejects the Hebrew Bible as untrue, one can suggest that the origins of Judaism as a monotheistic religion lay in rejection of polytheism out of some notion of moral, racial superiority. Or one can accept the Hebrew Bible as a legitimate source of historical information, which would lead one down a much more empathetic path. Remember that in the Bible, God commands Abraham to travel to a specific region for the purpose of establishing a home for His people. As told in the King James Version of the Book of Genesis (12:1-3):
"Now the Lord had said unto Abraham, Get thee out of they country, and from the kindred, and from they father's house, unto a land that I will whew thee:
"And I will make of thee a great nation, and I will bless these, and make they name grew; and thou shalt be a blessing."
If one takes the Bible literally, and many do, then a monotheistic Judaism was not created by people for their own advantage but rather born because God commanded it. Another history, the veracity of which is open to interpretation, is that of Flavius Josephus, a Roman-Jewish historian writing in the first century AD, who wrote the following with respect to God's covenant with Abraham:
"Now Abram...at the command of God went into Canaan, and therein he dwelt himself, and left it to his posterity.... [H]e determined to renew and to change the opinion all men happened then to have concerning God; for he was the first that ventured to publish this notion, That there was but one God, the Creator of the universe; and that, as to other [gods], if they contributed any thing to the happiness of men, that each of them afforded it only according to his appointment, and not by their own power."
Jewish law is ancient and inseparable from the Hebrew Bible. We could make the case that there were few to no advantages to Israelites in the development of a monotheistic, law-abiding religion. Indeed, given the long history of anti-Semitism (known as "the oldest hatred" of humanity), it is highly questionable whether any advantage accrued to the Jews as a result of their adoption of a monotheistic religion. The argument, given the history of the last several thousand years, could be advanced that the Israelites would have been better off following blindly in the footsteps of others rather than going on a separate path. That, however, would have required a rejection of the notion of a covenant between one God and Abraham.
If one has to make a case for the advantages of a conversion to Judaism in the time period, it could be argued that a significant advantage accruing to the Israelites by virtue of their adoption of a monotheistic, law-based tradition would be their presumed success in securing their place in the universe. The covenant entered into between God and Abraham would cement the Israelites' advantage in defeating their enemies and establishing a permanent homeland in the territory specified by God. In the Book of Genesis (15:18), the Bible states that God promised to Abraham and his followers a substantial expanse of land:
"In the same day the Lord made a covenant with Abraham, saying, Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Eurphrates:"
This covenant would be reaffirmed over the years, as when God addressed Moses, specifying the boundaries in which His people would live in peace. [Book of Numbers (34:1-13)]. By adopting, or accepting, the monotheistic religion that would become known as Judaism, the Israelites would be able to assert themselves among myriad enemy tribes and kingdoms secure in the knowledge that victory would be theirs. From a theological perspective, this is pretty significant, and it remains a religious foundation for many orthodox Jews today, even though many acknowledge the practical impossibility of any such development absent a major and highly visible sign from God that would be accepted by all those who stood in the way of the Israelites' determination to build a homeland within the boundaries specified by God.
By adopting a monotheistic religion, the Israelites would determine the word of God to be final; this would legitimize the Israelites' struggle for a homeland where they would be free from persecution. This, then, could be the principal advantage of a monotheistic religion.