That piece is basically a potentiometer that allows you to control the level. The 420 is a line-driver (boosts the signal to the amps so the amps play more efficiently and much cleaner), a dual-zone eq (tower and cabin separately), has a mic that only mutes the tower speakers when you talk through it, and has boat/tower speaker volume control, and a sub control. It also has an aux input like Todd spoke about.
Todd, I understand. I don't like it much either, but my ipods are set up by playlists, so to change all I do is press the up or down key. It actually works pretty good, and it works from the helm or the transom. It was worth the $50 for me.