What does the effective-from date do?

It says when the new value starts applying. Saving a cost closes the previous one the day before and opens a new one, so orders are always priced by the rule that was true when they were placed.

That means raising a supplier price today does not change what last quarter cost you. If you deliberately backdate a change, only the days from that date forward are recalculated. The mechanism is called effective dating, and every cost type in nouz uses it.