Perhaps if you are the author of the 20,000 line header file, but from my experience, this is certainly not the case when developing software in a team. Having

Perhaps if you are the author of the 20,000 line header file, but from my experience, this is certainly not the case when developing software in a team. Having all the code in one file is significantly more difficult to understand for a new team member since it becomes more cumbersome trying to understand the dependencies between different modules. On the same note, proper modularization of source files also encourages better discipline for dependencies among modules, such as discouraging circular dependencies.