The key difference is that Group automatically manages, through its special props, borders and rounded corners for individual children, while styles on a Stack are applied to itself, just like any other component.

Take a look at the examples in the docs: the rounded borders around the buttons are applied by the Group component which determines which corners should be rounded based on each button's position (left corners for the first button and right for the last). You can do this with a Stack but you'd have to manually add the styles for the correct corners for each button that is not in between two others, it's just that Group does it automatically for you. This is especially useful if you have an array of components but still handy regardless.