Our commentators take it for granted that this indicates a census – a survey of how many people the nation comprised. Moses needed to know the size of the nation.
Community leaders throughout history have had the same need and have counted heads for that purpose. In modern Jewish life it is more difficult because there are so many definitions of who should be counted in or counted out.
Rav Soloveitchik summed it up brilliantly when he spoke of two covenants – the covenant of faith (how many Jews are believers committed to the commandments), and the covenant of fate (how many Jews are part of the Jewish people even if they observe few of the commandments).
On both counts the figures today are rather worrying, though the orthodox camp at least is growing in both numbers and commitment.