It's how the internal "AST" for Markdown is represented in pulldown-cmark - a Vec of a node enum. A major motivation was reducing the allocation cost (it's extremely common to do one allocation per node for this kind of thing), but another benefit is that a tricky "splice" information for inline markup (basically, moving a sequence of text spans, possibly with their own emphasis, between two asterisks into a subtree) is pretty easy to represent.