I have paid for four outlining applications. Each was better designed than a stack of index cards, and each one lost to the cards within a month.
The reason is not nostalgia. It is that forty cards on a wall are visible all at once, and a book’s structure is a spatial problem before it is a written one. You can see a sagging middle from across the room. You cannot see it in a sidebar.
The cards also make reordering cheap. Moving a chapter is a two-second physical act with no undo history and no anxiety attached, which means I do it far more often — and the outline gets better precisely because rearranging it costs nothing.