It standardizes it across platforms and implementations; I mean you mention Moment.js, but it's been superseded by Luxon and DayJS ages ago; Moment is very large in terms of file size and doesn't support tree shaking, it creates mutable objects, same as Date, etc etc etc.

But there's the problem. Use momentjs today and you're behind the times, but use the new standard library date functions and you're pretty much guaranteed that code that works today will still work in 20 years.

One reason Temporal is a huge deal is because it differentiates between Date, Time, and DateTime. All of the libraries building on top of jsDate couldn't really do this effectively. I've been using Temporal in production now for about 2 years with a polyfill and while this distinction can be annoying at first, the ability to be so much more specific is very helpful.