Dynamics separate the men from the boys. Think of the song “Shout” without the “a little bit softer now... a little bit louder now” part. It makes the song. Or think how utterly un-souful a song like Led Zeppelin’s “Since I’ve Been Lovin’ You” would be without the amazing display of dynamics - vocally, how it just builds more and more throughout the song... the soft guitar solo in the beginning vs. the heavy one in the middle... and how Bonzo and JPJ back everything up perfectly. I basically forced my band to learn that song a long time ago as a masterclass in dynamics. It frustrated the hell out of them, and it took a good 3 months to be able to really pull it off. But when they finally got it, they got it, and they realized why I pushed for it. Because the band was never the same after that. And we had pretty good dynamics before, but that song took it to a whle ‘nother level altogether. Imagine Zep without great dynamics... Stairway... Dazed and Confused... Babe I’m Gonna Leave You. They would all be lifeless without the dynamics. It really made those songs.
And Zep is just one rock band. Think of The Doors “Gloria”. Hendrix “Hey Joe”, etc, ect... On the other side of the spectrum, look at a punk band like NOFX and how they are able to pull off so many styles and still sound like a punk rock band? It wouldn’t be possible without great dynamics. If I see a live band that has little to no dynamics I’ll usually lose interest very quickly, even if I know the songs. On the other hand, if I see a band for the first time that has good dynamics, even if I’ve never heard their songs before, they will usually draw me right in, usually regardless of the genre, or even how great their other musicianship skills are. Yup, dynamics are probably the last thing most musicians master, and probably the most important skill of all IMO. It conveys all the emotion.
"No dynamics" is just a wall of noise and doesn't rock. When a band comes out over the top loud, stays that way the entire performance and then ends that way, they are lame.