Mural is great for free form diagrams and sticky boards. Clickup/Notion/etc are good for nested documents. Whimsical lets you have stickies that can open up into a document, which can have other documents in it. But you can't nest diagrams inside diagrams.
I want to be able to think visually about items. If I have a list of development tasks, I want to convert them into stickies that I can freely move around and group. This would sync up backward. If I group stickies they become subtasks of a parent task. If I create a new sticky, it becomes an item in the table view as well.
But I can also collapse groups of stickies or drill into them to reveal more stickies.
I used something similar years ago in a BPMN modeling tool. You could have atomic process steps or steps you can drill into. When you click on one with a + on it, you were zoomed into that task, which had a sub-diagram inside it.