Pyret doesn't actually have an indentation-based format (so far as I know) it just happens to look like python.

That's correct. There is a preferred indentation (enforced by the emacs mode and our online editor built on CodeMirror), but whitespace is only needed to separate things - the actual amount of whitespace never matters.

So, for example, you can move chunks of code from one place to another and select it all and re-indent - something that you can't do (in general) in Python. It also makes it easier to programatically generate code.

When you have both braces/keywords and whitespace, it seems to me that you're essentially storing the same information about the program structure in two places, of which one is read by the machine and the other is read by humans. The "re-indent" operation reads the master copy and updates the other one. (And if you forget to re-indent, your human readers will be interpreting an outdated copy and be very confused.)

In Python, on the other hand, the information only exists in one place: the indentation. This is read both my machines and humans. There is no "re-indent" operation as the indentation is the only place the information exists, so there is nothing to sync.

I have heard people bring up programatically generated code before, but as another commenter put it "where Ruby usually denotes code blocks with a `do` and an `end`, Python denotes code blocks with a `:` and an outdent" – so is that really a problem? Isn't it just a slightly different way of encoding the same information?

(As to moving chunks of code in the editor: If the editor has a command to re-indent a selected block, it probably also has a command to shift it left or right.)

But if indentation matters, and it's pressent, then why do you need the colon?

Well, the colon can be used without indentation if the block that follows is just a single line:

  def embiggen(x): return x * 2
  for i in range(10): print(embiggen(x))

Also, it just makes things read more naturally to us humans, I think.