Documentation is the most important skill in computer science. When you write a thousand line of code, it is very hard to keep track of what you had written, and what is the purpose of the code. The image that in your personal libraries you have 100 files and in each file you have 1000 lines of code. There is no way a person can memorize and remember all of the code. It is much simpler to write the purpose of a specific code in terms of language, not only you can read it, other can also read it.
In the real world, big companies will have several team work on different part of the program. If the documentation is poor, when the team tries to put everything together it will cause many problems. It would be really inefficient since the company needs to take extra time to decode the code they have and use it for their own purpose. It will be much easier and efficient if the code were documented in the first.