The problem is that the expection was to document the software design in excruciating details (e.g. class diagrams with all fields and methods, etc.) before any coding. To the extreme you were meant to all the coding in your head and a Word document before doing it again in your IDE.

This wastes a huge amount of time and usually the documents become obsolete as soon as you try to actually run some code.

This is the very issue the Agile manifesto identifies and proposes a solution to when it says "working software over comprehensive documentation". 'Comprehensive' is a key word as they don't mean NO documentation but effectively just what's needed to plan and help people understand the code.