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.
Author here. Nbb is a scripting environment for Clojure running on Node.js. Clojure is a Lisp that emphasises functional and data oriented programming. Scripts are executed using an interpreter, so there is no build/compile step necessary, which is convenient for small applications.
If you're more curious, check out the README which has a list of videos and articles on this.
Nbb isn't a compiler, but a Clojure interpreter. Clojure is a hosted language, but can be compiled or interpreted. The ClojureScript compiler compiles Clojure to JavaScript, but nbb only interprets Clojure.
Basically, you take a programming language and make it work on a platform that meant to be programmed using a different PL. Clojure is hosted by design - it's not Java, but can be used to program for JVM. It ain't Javascript, yet can be used to target nodejs and browser; not an [official] CLR language, but you can write .Net programs. You can use Clojure to make Flutter apps with ClojureDart. You can integrate Python into Clojure with libpython-clj. Or write Clojure to target Erlang/OTP; or Rust; or R; There's even a clojure-like language for Lua - Fennel.
There's something about Clojure people like so much, they want it to work atop any platform.
Compiles/transpiles into code that runs on a "host" language execution environment (nodejs, JVM, CLR) instead of it's own VM implementation, or native code.