A person new to programming may be able to make a good small program.

However, when starting to work on anything bigger than a small (think 1 C source file or Python module) program, there are some general concepts which become much more important when working on "big" (think many Python modules or C files) programs; one example is modularity, another is having a set aim.

Some of these may be obvious to people who went to school to learn programming; however, people like me who did not go to programming classes sometimes have to learn these things from experience, possibly creating failed projects in the meantime.

==================================================

Please explain what the concept is, and why the concept becomes more important for big programs than by small programs.

Please give only 1 concept per answer.