Decomposers are important because if they weren't around none of the other animals in a ecosystem would die.
A couple examples of decomposers are soil, bugs, and fungi.
If these weren't around vegetive animals (which are normally the main preys) would all die because they have no food, and carnivorous and omnivorous would all die because they wouldn't have the decomposers or the other animals to eat.
Hope this helps!
-Payshence