Bacteria and fungi are called decomposers because they decompose Complex molecules into soluble organic molecules which are absorbed in the soil which will be later used by the plants again to fulfill its nutrient requirement. OR

Because bacteria and fungi break down the dead and decaying organic matter into simpler substances and provide the nutrients back to the soil. When an organism dies, decomposers decay the body extracting the last bit of energy from it. The common left over materials such as water, carbon dioxide, and minerals can then be reused by producers to make sugar through photosynthesis.

The advantages of decomposers are

1. They keep the environment clean by decomposing the dead animals.

2. It prevents the spread of diseases caused by dead bodies of plants and animals like bird flu.

3. They help in making humus.

4. They act as natural scavengers.

5. They help in the recycling of nutrients.