The biggest advantage if you can avoid significant whitespace is that it's usually a lot easier to manipulate text if you don't have to worry about the whitespace being correct. For example, when I'm copying and pasting Javascript code from somewhere else into a file, I don't need to worry about getting the indentation right, I just paste it into the right location, press format, and it does what I want. This is often nontrivial in Python, where both the source and the destination may have existing (invisible) whitespace. Worse, leading whitespace characters are often trimmed on copy or select, which often makes the resulting paste even more chaotic.
This is a minor inconvenience, and I still use Python regularly when it makes sense, but from a language design perspective, I think it's one of those decisions that makes your users' lives that slight bit harder with no real upside.
Select-lines-(shift)-tab when needed is quite easy.
Often that’s not needed either because moving from one function to another is the same level.
The benefit is not having to constantly read/write delimiters. Imagine a shell where all arguments had to be delimited by , or | chars instead of whitespace.
It's harder when the source or the destination isn't all indented in quite the same way, which happens often if you start a block, press enter and end up on an indented line, and then paste the code.
I don't really get the readability argument - like the lisper I was replying to said, it's all much of a muchness. A shell where arguments are delimited by commas is a function call in most programming languages, and people don't struggle with readability there. If anything, I find having an explicit "close block" symbol a useful visual marker on the page, but I write Python fairly regularly and don't really notice much readability differences compared to any of the brace-using languages I work with.
I configure my basic editor to remove trailing whitespace on save, not specifically for that reason but it does mitigate it.
The point was that whitespace was always significant. People like to pretend it isn’t for some reason. Perhaps because it has a minimum though not a maximum.
There’s a reason Python is considered very readable and “executable pseudocode.” Less delimiter and sigil noise is the main reason.
This is a minor inconvenience, and I still use Python regularly when it makes sense, but from a language design perspective, I think it's one of those decisions that makes your users' lives that slight bit harder with no real upside.