Why must products intended for residual weed control remain active in the upper soil layers?

Answer

To be effective against new weed seeds attempting to germinate

Residual products are specifically designed to remain active in the soil environment for weeks or months because their purpose is to prevent weed seeds from establishing new growth.

Why must products intended for residual weed control remain active in the upper soil layers?
environmentpersistencesoildegradationtoxin