The main problem is that documentation should be written on why code is written the way it is, and why it exists in the first place. This context is typically

The main problem is that documentation should be written on why code is written the way it is, and why it exists in the first place. This context is typically not available in the code itself. In the best case, this is encoded in requirements and design documentation, but more often than not, the information remains only in the heads of customers, architects, and developers. The code itself merely describes how something is solved. Summarizing that in documentation can be useful at times, but it is not the full story. Especially for code that lives on for some time, the original design philosophy is often lost, and it is forced in horrible directions.