I've edited the title slightly to make the intent of the question more obvious.

I wish to define methods within the class they belong to. Using class << self demonstrates that approach clearly — we are defining methods within the actual singleton class scope.

Explicit affordances are obvious, perceptual features of an item that clue you in on how it is to be used. With explicit affordances, physical appearance and any accompanying language or text inform the user of how an object is to be used.

I like this approach more because I can scan the code that renders the Box component and easily spot that it takes two children. If the Box took any props, they'd be within the opening <Box> tag, and they would be distinct from any children props.

Instead of trying to educate everyone on the complexities of copyright law, we’d rather make our intentions clear with this simple statement: