As I said, look at them as a compass - a map will be a better analogy maybe - this is what I want to achieve at the point in time I'm making my plans, and here's how I'm going to achieve it. As long as that doesn't change, it's useful to break down a vague goal into milestones and evaluate on occasion (my team does it every 2 weeks) how you're progressing.

But it's okay for that to change, and then you just write new OKRs.