If you need to build a whole game, you have to break it into smaller pieces, and then those smaller parts into smaller parts. This is one of the most important parts of programming you can learn. Modern operating systems are hyper complex, but they can be replicated (please don't attempt this until you are much older and have more letters after your name.) if you break it into smaller parts.
Successful scientiest are successful becuase they can replicate things. You want to be able to do this in computer science as well.
As a matter of fact in coding it may even be more important because someone will likely need to understand what your code does at some point. If you leave clues the whole way about what everything does, it makes that person (whether it is future you, or a co-worker) able to understand and work with your code.