The first instruction given to Moshe in Sefer BaMidbar is to count Bnei Yisrael. This is not the first time they were counted. In the Portion of Ki Tisa Moshe was also told to count the people. There is one major difference in how the counting was to be carried in this Portion and the previous one. This new counting was to take place in the presence of representatives of the tribes.
Hashem tells Moshe that among those taking the census there should be: אִישׁ אִישׁ לַמַּטֶּה, “…one man from each tribe…”. (Num. 1,4) Why was it necessary this particular time to have representatives of each tribe present? The famous commentary of the Malbim deals with this question. He claims the answer lies in the purpose of this census. This time the exact number of members of each tribe had to be known.
When the Jews were to enter into the Promised Land the division of the country among the different tribes was to be made. The land was to be divided according to the size of the tribes. Each tribe was to get a portion in accordance with its population. This count of the people was to determine how large a portion of the land was to be given to each tribe. Hence it was necessary that there be delegates representing the tribes so that there be no grievances or grumbling later that the figures used were inaccurate.
For people and countries to live peacefully, there must be complete understanding and accurate information so that no suspicions can be had.