Decomposition is breaking a large problem down into smaller tasks. Splitting a problem up is worth the effort because the smaller tasks are far easier to solve than tackling the whole thing in one go.
When you work on a small task, you can hold everything you need to know in your head at once. Imagine you are writing a program for a school sports day. It has to store each runner's time, work out the fastest runner and print the winners. As one job that is a lot to think about. Broken down, the tasks are just "collect the times", "find the fastest time" and "print the results", and each one on its own is straightforward.
Because the tasks are separate, they can be handed to different people. One person writes the part that collects the times while another writes the part that finds the fastest time, so several people work at the same time and the solution is finished sooner.
Focusing on one small part also means you do not spend time analysing detail that is not relevant to it. The person finding the fastest time does not need to worry about how the times were typed in. Each small part can be tested on its own, and when the tested parts are combined into the single larger solution it contains fewer errors than if it had been written in one go.
Edexcel GCSE Computer Science · Abstraction and Decomposition