I also find square brackets useful for other kinds of expressions, like Python-style list comprehensions. Though such syntax predates Python, e.g. there's a paper from 1991 https://3e8.org/pub/scheme/doc/lisp-pointers/v4i2/p16-lapalm... whose code still works in Common Lisp. If the language doesn't mandate that square brackets mean something, as Clojure does, then I can make them mean what I want that is convenient for my namespace, as Common Lisp allows. Whether that's data literals or special syntax or something like [x (x <- xs) (oddp x)] to filter out odds, it's nice to have that choice.