Clojure/Clojurescript is a functional lisp dialect and therefore brings you more and other ways to write programs, since it defers much from javascript/typescript. But I think another question would be "why do you need nbb for clojurescript?". The minimal answer is: clojure has slow startup time, and clojurescript need some tooling to produce runnable code. With nbb (or bb for clojure) you can run Clojurescript as a scripting language in the terminal, like python or shell, and still use some libraries both from the nodejs or clojure(-script) ecosystem.