In traditional statically-typed OO language, you would create class once, and never modify it again. What is the most important thing of that class? A name. You'd have to think really hard how to name this class, or else it will be the source of daily WTFs. I guess, that would be extremely hard.
The obvious benefit is that you never break anything that depends on your code. This enables you to write new code without fear that you may break something - which is very serious problem in software. This is my sand castle, a code that is as flexible as language allows it.
Now look at typical software. Each line of code is modified 10s if not 100s of times. Often changes have cascading effects. Code is full of unexpected behavior where names do not reflect the actual behavior, or even intent.
Why is it so hard to write code following OCP? I think, it is because we do not know what we are doing. Software we are supposed to create is beyond our comprehension. And so, we try creating random things, until something works as we thought it should. We may call this TDD, where we code to examples, and not full specification. On the other hand, there can be no specification other than code itself. The set of what -> how mappings is so big and with so many interesting options, there is no way we can choose The One, The Right option.
