According to the division of the land and the laws of inheritance introduced in this week's Torah reading, a man's land and goods were to be passed on to his sons and divided among them. There was an Israelite named Zelophehad who had four daughters. Because he had no sons who could inherit land, his family was not going to receive any portion of the tribal allotment of Manasseh. Not only would that leave his four daughters landless, it would mean that his name would not be remembered in the tribal allotment.
Zelophehad's daughters realized that they had to do something. They decided to petition Moses with their case.
Our father died in the wilderness, yet he was not among the company of those who gathered themselves together against the LORD in the company of Korah; but he died in his own sin, and he had no sons. (Numbers 27:3)
When the daughters of Zelophehad asked Moses for a ruling, he did not decide the case himself. He brought the question to the LORD. Why did Moses need to consult God? Why was he unable to judge the case himself? Rashi suggests that the ruling escaped him because God was gently chastising him. When Moses appointed judges, he had told them, "The case that is too hard for you, you shall bring to me, and I will hear it" (Deuteronomy 1:17). God used the case of Zelophehad's daughters to remind Moses that some cases were too difficult even for him.
This should remind us as well that there is no virtue in answering a question when we are uncertain. Rather than admit to being ignorant on a certain subject, people often bluff competence. Clergy and Bible teachers tend to be the worst offenders. Their congregations and students look to them as spiritual authorities. Because of that, they pepper them with questions. Rather than appear uninformed or uncertain in a certain area of knowledge, the Bible teacher will provide an answer off the top of his head. The sages say that a wise man answers a question to which he does not know the answer by saying, "I do not know."
The question proposed by Zelophehad's daughters constituted a difficult matter to which Moses did not know the answer; therefore, he honestly admitted he did not know.
The Chofetz Chaim offers a different explanation. He says that the question was not legally difficult for Moses, it was morally difficult. Moses felt unable to settle the case because of the way that Zelophehad's daughters presented it to him. They said, "Our father died in the wilderness, yet he was not among the company of those who gathered themselves together against the LORD in the company of Korah" (Numbers 27:3). When Moses heard these words, he realized that he could not serve as an impartial judge in deciding the case. He would be inclined to rule in favor of the daughters of a man who had remained loyal to him. Therefore, he removed himself from the case and brought the matter to God.