Even code you write yourself, given enough time, you will forget the why unless you wrote comments. In a way comments are as much for you as they are for others.
Even before AI, understanding code you didn't write is essential to working on a team of other developers. If you can't understand the code from reading it, then that's part of the feedback loop - too complex, needs comments, etc..
On large teams you'll spend as much time reading code as you do writing it. And long term when it comes to writing maintainable code - the ability for others to read and understand it, including the why of it, is paramount. Your code could literally be around for decades.
Code is never missing contexts. If what your code is doing is not obvious to the reader, it is bad code that needs to be fixed. Things like cryptic low-level expressions should be extracted to helper functions with descriptive names or even extracted into a class, and classes need to comply with the single responsibility principle.
No, not really. You get spaghetti code by being unable to refactor your code to follow inconsistent level of detail across calls. That's the textbook definition.
Once you start to follow basic code quality and software engineering principles, you'll notice right away that your code becomes both easier to understand and to test.
And if you already know the material explained by the book, yes i don't need to write it to understand it.
As a result there's a whole universe of code where the how of it, the elegance, is the main thing, and what it's doing is putting characters on the screen a bit slower than the next thing but there are some amazing concepts that are supposed to make it all an axiomatic synthesis of how to think about code forever, replacing all precious concepts of thinking about code.
Now AI can think about code forever while doing nothing.