Both the phono input and the line input refer to two separate stages of the amplifier. The former will be of use to those using a (perhaps older) turntable that requires its own external preamp stage to boost the weak audio output. For those using a turntable with a built in preamp or any other audio device that has been preamplified on its own terms, then the line input will be most useful, for it does not seek to amplify the signal any more than it needs to.