It doesn’t need to; that’s just the section numbering. In a biology textbook, you could have something like (made up names)

  Arabateciae
    Arabaticopoda
    Arabaticoquasicae
  Bidoroca
    Pseudobidoroca
    Superbidoroca
      Superduperbidoroca

it would look better still with a dash or a bullet point for every sub-entry. We're not arguing that it is possible to do that, we're arguing what is ideal for readability.

In that list you can naturally guess what that ordering is, but if the items were not so interrelated it can be confusing. if the top level item is 'Ham' and the indented item under it is 'sandwich' are you wrapping the same phrase 'Ham Sandwich' , because indentation (even in python) is used when wrapping lines, or is sandwich under ham as one of the things done with ham. it is thus error-prone and more confusing, clear and specific punctuation alongside indentation makes it easier to read.

No one is saying that indentation can not be used to display lists/sublists, I'm saying that markers remove ambiguity even across movement of blocks of texts.

Indentation is fragile.