Why do farmers normally use a …

As the seeds germinate, the growing saplings should be provided with nutrients and water. Hence soil should be replenished with minerals in periodic intervals of time. Manure and fertilisers supply nutrients to the soil.  This is called as replenishment of nutrients.
Manure and fertilisers replenish the soil by supply nutrients to the soil.   
*Manure is the organic matter added to the soil to replenish it with nutrients. The wastes from household, animal dung and agricultural wastes are buried in small pits. Decomposition by bacteria and earthworms occurs in these pits converting organic wastes into manure.   Manure increases the water retention capacity of soil.Manure is ecofriendly and replenishes the soil with nutrients which can persist for longer time whereas fertilisers though not ecofriendly supply nutrients for immediate usage by the plants.
*Fertilisers supply specific type of nutrients for the plant. Fertilisers are the chemicals which add minerals like potassium, phosphorus and nitrates to the soil.