There are at least a three lessons that I have observed while revisiting the book of Numbers.
First, past preparation is no guarantee of future success. The Israelites made extensive preparations to enter the Promised Land in 1:1–10:10 and yet they failed to do so by making a forty-year mistake at Kadesh Barnea (13:1–14:45). This is a point that I try to drive home to my seminary students. All their ministerial preparation could be brought to nothing by failing to trust the Lord.
Second, yesterday's obedience no matter how frequent can be nullified by today's disobedience no matter how rare. I count over fifteen statements in Numbers 1–10 related to the obedience of either of Moses, Aaron, the Levites, or Israel (e.g., 1:17-19, 54; 2:34; 3:16, 42, 49-51; 4:34-37, 41, 45, 49; 8:3, 20, 22; 9:5, 19-23; 10:13) and yet a disastrous decision at Kadesh Barnea made all that obedience for naught.
Third, God's purposes will prevail whether in the current generation or the next. In Numbers 26, we begin the preparation for the next generation, the generation that will enter the Promised Land. This is both solemn reminder and spiritual goad.