Except when you've worked on building the software yourself instead of getting the LLM to do it, you have a loooooooot of built-up context that you can use to know why decisions were made, to debug faster, and to get things done more efficiently.

I can look at code I wrote years ago and have absolutely no memory of writing it, but I know its my code and I know where some of the warts and traps are. I can answer questions about why things work a certain way.

With an LLM, you don't get that. You're basically starting from scratch when it comes to solving any problem or answering any question.