"Statements are executed but don’t produce any result value ... Expressions evaluate to a single result value ... A spread (`[i, ...thisIsASpread, j]`) is neither of those. A spread can evaluate to zero values (if you spread an empty collection), one value, or many. It’s its own kind of thing."
The last part of the article is what comes out of this new-kind-of-thing when you add it to a language as a holistic concept.