> There are many cases in which I already understand the code before it is written. In these cases AI writing the code is pure gain.

That's only true if the LLM understands the code in the same way you do - that is, it shares your expectations about architecture and structure. In my experience, once the architecture or design of an application diverges from the average path extracted from training data, performance seriously degrades.

You wind up with the LLM creating duplicate functions to do things that are already handled in code, or using different libraries than your code already does.