Rock and roll has always about getting attention at the end of the day. Regardless of how good you think that your new record is, it ain't going to mean a thing unless you have some sort of conviction behind your delivery. Even though plenty of artists like to just let the music do the talking, it's sometimes a little better if you have a singer with some charisma.
Ever since the dawn of the rock star, people have always expected a little more from the singer. As opposed to just the guy who sings the songs you all like to hear, it's always important to have someone up there who will put their heart and soul into every word that comes out of their mouth. Rather than just do the regular job of a singer, these vocalists know how to hold their audience in the palm of their hand, with every single person hanging on every word that comes out of their mouth.
Whether it's using gimmicks or feeding off the endless amount of stage presence they already have, these guys could be considered superstars regardless of any singing they've actually done. While the riffs may be the most important part, the great rock and roll needs to hold your attention, and these guys know how to grab you by the throat.