Why did the Lord inspire men to write the scriptures? God always has a purpose... even if we may not always understand it. The numbers and the organization themselves have a purpose, part of which certainly, is let us know (the natural Israelites as well as you and me) that God does have a plan, which includes every necessary detail. He has always had a plan for His people. We certainly should desire to be a part of the positive part of that plan. His Word, if we can understand it, tells us how to be part.
God knows all things, but we do not.
"O LORD, I know that the way of man is not in himself: it is not in man that walketh to direct his steps." Jerem 10:23
And do we really own anything before we inherit? God owns the cattle on 10,000 hills, but we own... No, we are stewards or heirs to be, but we own nothing... yet.
We count what the Owner requires us to count. Are we paying attention to what God asks of us or what we want?
I think it was for organizational purposes, as they would be setting forth on their journey, and required organization for their encampments and also in preparation for battles (how many men were fit for battle), and to demonstrate to Moses and the people the Lord's faithfulness in multiplying the seed of Abraham. God is a God of order, and His people were not a rabble but a highly organized group.