The numbers on the tabs are for counting. They enabled the postal clerk to quickly remove a set value from the sheet.
For example, in this sheet, each stamp is valued at 12+38 Pf, or 50 Pf.

The numbers in the top margin show the value of the sheet to that point (working left-to-right). In this instance, because 100 Pf = 1 Mark, the first column has 2.50 Marks of stamps. If a customer wanted 10 Marks worth of stamps, the clerk would know instantly to remove 4 columns of stamps to produce the desired amount.
As far as value goes, there is a slight premium for having the margins attached.