When using cm3 as the unit for volume to calculate the concentration why must you divide the cm3 by 1000?

As I’m sure you already know, the units of concentration are mol dm^-3. So before we calculate concentrations we must convert the volume with units cm³ to dm³. 1 dm is the equivalent of 10 cm.
So, by cubing these values we get 1 dm³ = 1000 cm³    which is the same as    0.001 dm³ = 1 cm³.
So in order to convert cm³ to dm³ you simply have to divide cm³ by 1000.
In the opposite direction, if you have a concentration of mol dm^-3 and want to find the volume you will have to multiply by 1000 to get the units cm³.