At its heart, Rock Band is a celebration of music and its ability to bring people together, which certainly helps explain why it’s so much better in a group setting.
Considering most people have likely played a Rock Band at this point, there’s really no point in discussing what makes it so good, but rather why. Although the plastic instruments genre has largely died out, franchises such as Rock Band still make for great party games, at least if you managed to hang onto all of those bulky peripherals. Still, there’s no reason to dig out Rock Band unless you have some friends play with, as the series becomes infinitely better when enjoyed with a group.