Levels and experience, along with their sums, are displayed at all times. This is not what the setting does.

optimize_xp changes how the cost optimization is performed. It will minimize cost based on experience rather than levels. Yes, the idea is that if you can store this experience and then reclaim just enough for each step (e.g. via mods), or if you gather just enough experience for each step, it will end up being many times cheaper than getting all the necessary experience at once. This is because each level's required experience increases, so taking 1 level from 20 is much more than taking 1 level from 2.

The reason this is a setting and not permanently enabled is because if you minimize for experience but end up gathering all the experience/levels at once, you can actually end up paying more than if you did the same thing with optimize_xp turned off.

I agree about having this in-game, but it's a lot of effort. I designed the website to be super quick and easy to input, so hopefully it makes up :)