Most editors nowadays maintain a parallel structure to the DOM. And since the DOM is a tree, that structure is also a tree. Quill's is Parchment[1] and goes a step further by also having an API and allow defining new nodes, which ProseMirror (and maybe Draft) also does.
Deltas are a simple, iterable output format for Quill. In practice it is much nicer to deal with this in many use cases than the Parchment tree.