I'm not sure it's possible to write a piece of software that only you understand, that is of high quality (works reliably). I feel like code correctness (what a computer does with it) and readability (how well a human understands what is written) are two sides of the same coin.

I think I would argue that if someone has happened to write a program that is impossible to understand for human readers (unreadable), and yet it does exactly what it's supposed to do (is correct), at the very least this program will break when the author starts to refactor it, which always needs to be done at some point.

I agree that conveying an idea to another human being through code is useful, but I think the number of times a human being has written a piece of computer code only to convey an idea to someone else is fairly small. If I want to convey an idea to another person I write in words and concepts, but if I want something faithfully executed every time, I need to write the code. And perhaps someone will look at this code later, but the origin of the code was always to get something done, not conveying an idea to someone else.