A dual keyboard setup! Unnecessary? Perhaps. But it does prevent you from straining if you are typing ‘at’ something on the second monitor, which you want to look at whilst typing.
It’s also useful if you want to have something open for reference and glancing at up on the white MacBook screen (*cough* Twitter *cough*). Just don’t need to type anything on that machine. And if you do, you hook up the wired Apple keyboard to that machine instead.