If you imagine every block as a separate note, that’s what Roam means when it says the block is the core element.
In Obsidian, if I have a paragraph in Note A, I can transclude that paragraph into Note B. If I delete Note A, I also delete the paragraph. In Note B, the translcusion is broken. In Roam world, Note A isn’t one thing. It’s an outline of blocks. I can delete any of the blocks without affecting the other blocks.
You can get this same effect in Obsidian if your notes were super atomic: every heading or paragraph or list item was a separate note and larger “notes” were created by transcluding a bunch of other notes together.
Most people don’t need Roam’s super-atomicity. For those that do, Obsidian’s transclusion is too clunky, not fine-grained enough.
(Begs the question, what kind of workflow really benefits from that level of atomicity?)