These layouts provide several additional layers which give easy access to ASCII special characters, greek letters and frequently used math symbols.
Also there is layer 4, which is pure gold in itself. It provides cursor movement keys, backspace, enter and a numpad close to your home position. I personally couldn't go back once I had tried it.
(Beware that these layouts include the umlauts äöüß as they were designed for german writing, and make use of the additional key for '<' in the lower left corner of german keyboards)