In reactive notebooks, the program state is still being completely described by the code you see - it's just that it doesn't have to be in top-down order! Instead of forcing you to manually define everything before use, a reactive notebook just looks at the references you make, forms a dependency graph, and then evaluates it in topological order (and, as optimization, only re-evaluates things that depend on the thing you've just changed).

This leads to a much nicer code organization, particularly when you're writing something resembling an interactive document (e.g. an "explorable explanation"). Much of the documents I do on ObservableHQ has roughly the following structure:

  Title
  Prose
  Visualization
  Interactive UI elements (possibly mixed
    with further visualizations)

  ------------------------------
  All the actual source code
  powering the implementation,
  organized in readability order.