One of the easiest ways to understand the logic behind the modular design is to look at LEGO blocks. We can say that all of them are individual modules that can be combined together to form a system. After you've done it, it is easy to replace some of them without breaking the whole system down. It is the approach that lets you make changes on your own. If some of the popular electronic devices were made using it, there would be no need for us to turn to manufacturers for repair. As you can guess, they do it on purpose. The opposite way of creating a system is a homogeneous approach. Monolithic structures are then used. If the modular design is so convenient, where can we use it?
It can be of great help during the problem-solving process especially when it comes to massive and difficult issues. Instead of looking for one person with outstanding and unique skills, you could break the problem down into several tasks and ask different people (or teams) to find the solution for each of them. They do not even have to know about the existence of that big problem. It's enough for them to focus on their particular task.
One more feature of a modular design that lets you use it effectively is the possibility to reuse and recombine its units. When you have a list of modules, you can create various combinations. There is no need for building something completely new. Just rearrange the building blocks you already have. Once some of the modules wear out or are damaged, you can replace them with ease. This concept also applies to the customization approach. The automotive industry, for example, successfully uses it letting customers decide what features their vehicle should and should not have.