! might be "less" punctuation than [[ noreturn ]] but is a lot harder to google. And why do you need the "->"? That is just more line noise.
In theory, punctuation is noise. In practice, it helps you understand the parts of a sentence. Same with code.
Also because "no return type" is the most common return type, and this way representing "no return type" as omitting the `-> ()` reads nicely: `fn foo() {}`
I think that the advantage of leading with `fn` arrives when you have paragraphs of functions to scan through.
Leading with a consistent word (be it fn, func, or whatever) immediately and disambiguously identifies the line as defining a function (rather than a variable, or whatever), which is a useful landmark to anchor on or pivot from.
Clearly this doesn’t matter if you’re in an IDE with a working “find definition” feature, but that’s often enough not the case that I find easily greppable definitions to be a gigantic advantage.
To be honest, type ascription being : was a (small) mistake that affects the comprehensibility of errors because it is distance 1 from multiple common typos and is valid almost everywhere in the grammar, so it is very easy for the parser to go down the wrong route. Even worse, the feature is nightly only so we're not even getting the benefits of the simple syntax. Advice for language designers: make your grammar redundant, with lots of sign posts for the compiler to recover when things are wrong. Things like mandatory semicolons are good for both humans when reading code and for the compiler when parsing it. You can also make your compiler perform semicolon autoinsertion, but still have an error, that way the type checker can still run.