Pre amps send a low-level signal that an amplifier can use. If you were going to use aftermarket amps, or use a subwoofer, then you'll be using the preamp connections. CD-players with 3 sets of preamp outputs are especially nice because they genereally have a built in low pass filter that allows the subwoofer to play just the low frequency sounds (IE: 120hZ and lower) that setup would also allow you to adjust subwoofer volume independently of the music volume. Some stereos with 2 preamps will allow this feature as well tho. 3 preamps = 1 output for the front speakers, 1 output for the rear speakers, and one output for the subwoofer(s). If you plan on using the stereo's on-board amp to power the speakers, then those features won't necessarily apply to you, but they will give you more room to grow without buying another stereo.