Is there a rationale or even benefit behind this syntax, or is it just syntactic sugar for the $obj->some_method notation?

Indeed, your second code example is demonstrating 'indirect method invocation' by way of using an 'indirect object', which is syntactic sugar for all the C++ (and the like) types. For more info on the subject do a search for 'indirect object' and specifically see tye's node on why it's not generally a great idea.
HTH

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broquaint