There's many of us out there who'd never even heard of Diamond Head before Metallica started covering their tunes. There's also plenty of people who didn't know the song "Am I Evil?" was actually a Diamond Head cover.
And let's face it, what metalheads out there gave a sh*t about the Misfits before Metallica covered "Last Caress" and "Green Hell"? After that, you saw Misfits shirts at metal concerts everywhere.
When Metallica's Garage Days Re-Revisited E.P. came out in the summer of '87, it was enormous fun to listen to. Even though I hadn't heard any of the original versions of the songs, (although I knew who Killing Joke were from seeing their video for "Eighties" on MTV), it was clear what Metallica took from these bands, and how it helped form their own sound.
Metallica always stressed the importance of covers in the journey to creating your own music, and the bands they covered were so obscure, many thought they were originals when they saw the band live. (Metallica's first live show was three original songs, and seven covers.)
"We started life as a cover band," Lars Ulrich told David Fricke. "It was just, 'Let's do something fun. Here are these records we sit around and listen to, here are the records that get us off. It would be fun to play some of this music.'"
And as Stevie Ray Vaughn pointed out, he wasn't trying to take away from the blues artists he loved when he got famous, he was trying to give back to them by showing how much their music meant to him. This is certainly something that hasn't been lost on Diamond Head guitarist Brian Tatler, who recently told Radio Metal the band "would've faded away in the late '80s without Metallica." (They've also thankfully made a lot of money from Metallica covering their tunes, which had to be nice after years of struggling.)
As Radio Metal mentioned to Tatler, "without Diamond Head, no Metallica, and without Metallica, no Diamond Head," and as he just mentioned, they wouldn't be here today without Metallica covering their tunes, but there's still confusion among fans as to who first did songs like "Am I Evil?"
"I don't mind, because I get the royalties!," Tatler continued, although he's had to tell fans "to look at the small print on the record where it says, 'Written by Tatler / Harris in 1980.'"
One of the better known bands Metallica has covered and paid tribute to is Motorhead, one of the founding fathers of speed/thrash/ extreme/ whatever you want to call it metal.
In the early days, underground metal fans felt Metallica sounded like a combination of Diamond and Motorhead. As James Hetfield told David Fricke, "We put the two 'heads together, and came up with something unique."