Writing code is a process of getting your understanding of the requirements, and the thoughts of how to implement those requirements, out of your head in the form of code that's written in the programming languages that you prefer. Creating design documents is a process of outlining solutions that you have in your mind in a form other than code, for example, UML diagrams. Essentially, code and design documents are the media that represent your thoughts.

Design documents are useful because they are the outlines of your solution. You can use them as guidance through code writing. And during writing code, usually, you can get inspiration for the solution. New ideas will come up or you will find flaws in the outlines. Eventually, the solution is written into the source code. From a product perspective, the only thing that is used to create the final product is also the source code, which is built into zeros and ones by computers. From this angle, you can think of computers as the workers in a factory, and the source code is the design they use to create products.