It does[1], but it's not necessarily preferred. The whole point of Typed Racket is that you can write your program in (untyped) Racket originally (maybe starting out as a small script), and then add types if you need your program to be more robust. Since typed and untyped modules interoperate smoothly, you choose whatever combination works for your project.

That said, some libraries start out typed too (e.g., the Racket math library[2]) and do benefit from type-driven optimizations and type-checking.

[1]: http://docs.racket-lang.org/ts-guide/ [2]: http://docs.racket-lang.org/math/

Typed Clojure is in fact based on the research behind Typed Racket.