you can rename a CSS file in .scss and it will just work
Because SCSS is compatible with CSS, it means there is little to no learning curve. The syntax is already known: after all, it’s just CSS with a few extras. This is important when working with inexperienced developers: they will be able to quickly start coding without knowing the first thing about Sass.
Moreover, it is easier to read because it actually makes sense. When you read @mixin, you know it is a mixin declaration; when you see @include, you are calling a mixin. It doesn’t make any shortcuts, and everything makes sense when read out loud.
Also, most existing tools, plugins and demos for Sass are developed using the SCSS syntax. As time goes, this syntax is becoming pre-eminent and the default choice, mostly for the above reasons.