What is the importance of decomposer?

Answer 1

Without decomposers organic detritus would build up to a level that would bury the earth in dead organic compounds. Think what would happen if in the fall all the dead annuals just kept piling up tear after year with decomposing. Soon nothing could grow because all the carbon would be looked up in this detritus.

Answer 2

Well if there were no decomposers dead things would just keep piling up so they are very important.