Root selector vs Element selector

I have a question about the 2nd design project, the survey form. In the form they use a :root selector with the colors defined. So my question is why would you use that instead of using just an element selector with the color instead in the style sheet?

Imagine you have a huge website that follows a colour scheme, you style all your <h1> to <h4> in a certain blue colour, then also all buttons, and maybe the background colour of a few sections also get that blue. If you want to change that colour, you’d have to change it for all selectors throughout the whole CSS. But if you give them the variable $blue instead, you change that colour once - at the top in the :root.

Makes a lot of sense, just enter the selector and enter the variable and done. Got it thanks!