Improve Compatibility with Browser Fallbacks, What is its advantage?

Hi everyone
already I know the solution of this challenge “Improve Compatibility with Browser Fallbacks” by adding this code: background: red; to .redbox class.

but what is I don’t understand is:
If we improve our browser compatibility by adding another background declaration right before the existing declaration and set its value to red, then using css variable has any advantage yet?

More recent/modern browsers will use the later statement, but in the example used I’m not sure what the point is, either. If you’re going to have to set the colour separately every time it’s pointless using the variable.

I’m not sure what a better example would be though to teach the idea behind it.