If your intense orange button has nearly the same lightness as its intense blue background, it’s going to look like a headache-inducing blob instead of like a button.

> light orange button against a dark blue contrast creates a far stronger separation than a light gray button against a dark gray background

It’s not that much stronger separation, in my experience. Light on dark is much more fundamentally important to the design than orange on blue.

If you made it intense middle orange on intense middle blue, the result would be horrifically bad. But light gray on dark gray is just fine.

If you get the lightness contrast right, you have broad latitude in color choices and you won’t be able to screw things up too badly. You can switch between light yellow on dark red, light blue on dark purple, light orange on dark blue, light gray on dark brown, etc. On the other hand, if you get the lightness contrast wrong, then there’s pretty much no way to fix it with color. Which is why it is helpful to design for lightness first.

That doesn’t mean that you can’t e.g. keep in mind while you are doing your black and white setup work that a particular button is going to get a super zappy red color against a largely neutral slate colored design (or whatever), but even so, you want the lightness relationships to be right. Doing the lightness work separately up front can be helpful for many artists: it will preclude many of the worst design mistakes, and focus attention on making text legible, structurally important shapes stand out from their background, and so on.