Earlier quoted context omitted.
> It's not clear to me what it does just from reading it How isn't it entirely obvious? := is the assignment operator in tons of languages, and there's no reason not to have assignment be an expression (as is also the case in many languages).
> := is the assignment operator in tons of languages It is? Which ones? Other than Go, I can not think of a single language that has ":=" as an operator. Java does not, JavaScript does not, C/C++ do not, Ruby does not, I don't think PHP does, Erlang/Elixir do not, Rust does not... (I could be wrong on these, but I've personally never seen it in any of these languages and I can't find any mention of it in these langua…
The operator was inherited by Pascal, Ada, Delphi. That line of language syntax died off in the late 90's though, so I can see why younger (and in particular self-taught) programmers wouldn't be familiar with it.