Just as a director takes plenty of time agonizing over the first shot, they will also take great pains to make sure the entire first scene is ripe with meaning.
So why do we spend so much time (there's that word again) concentrating on all those dials, and on Marty blowing himself backwards by the force of the amp? The symbolism here is twofold.
First, it gives us some insight into Marty's aspirations. He wants to explode onto the music scene—here he quite literally experiences a musical explosion. His band is later criticized for being too loud. Yeah, Marty is all about going big or going home.
But it's also an indication of Marty's irresponsible tendencies. He makes a mess out of just about everything later in the story, and the literal destruction he causes here is symbolic of his later exploits, such as preventing his parents from getting together, or his ill-advised fiddling with fate when he writes Doc that letter.