There are many beneficial, naturally-occurring bacteria in soil. These bacteria break down the compost into a nutrient that plant roots can absorb. This is the plant's source of food. The healthier a plant is, the better it will be able to resist disease and pests.
Watering your garden with chlorinated water can kill microorganisms in the soil, including these beneficial bacteria. Using a dechlorinating garden hose water filter will protect these beneficial bacteria from the harmful effects of chlorine.