theoretically, you could just use photoshop (or keynote, more realistically) to design interfaces. however, given that from there someone has to take over and implement it using the real technologies, it is easy to miss things and the process becomes hopelessly slow. i guess it is theoretically possible to operate like this.

contrast that to being able to rapidly iterate on the real code.

i don't mean iterate as in "code a feature, commit to the repo, send for review, do over". i mean the tweaking while coding.