Selectors are dynamically generated => don't have to remember to remove them when removing CSS and vice versa.
Selectors are combined and reused => .color:red,.button{color:red}.
Selectors are minified from long text-align:left to short a.
You open a template file and a browser and you just type the selectors. You don't have to switch between HTML and CSS for removing, renaming, and manually combining classes.
The CSS size doesn't grow exponentially, because there is a minimum of duplicated CSS property:value. There is some article about CSS size from Facebook.
When frontend and backend engineers work on one task, then it's easy for the frontend engineer to tell the backend engineer which classes to add when he just needs to indent or align something. He doesn't have to edit CSS and hopes it will work. He just copies selectors.
Dynamically generated CSS means fewer files in the projects => fewer changes => we normally generate cache, entities, migrations, js bundles and etc. So Stylify dynamically generates CSS. At least from my point of view, it's more comfortable and efficient than writing it manually and thinking about how to pre-generate utilities or combine CSS files manually.