That's actually not true. The Closure compiler has certain restrictions on the Js it accepts for advanced mode compilation, so you can't throw just anything at it.

The advantage is best-in-class dead code elimination, which is also why Closure was chosen; to distill the CLojurescript standard library into just the parts needed for a web page requires good DCE. Recently, other Js projects like Rollup and Webpack have gotten good DCE, but when Clojurescript was started, there were no other serious contenders.