And now I’m wondering, who actually promoted the bands that got lots of attention in the recent years? Was it only their labels? Was it luck?
Most of them were promoted by the relatively small group of PR professionals who have a track record and good experience with labels, artists, booking agents, and bands.
PR agents let people know when music is being released, when a band is touring, etc. They can't make them care about the music. There are plenty of PR people/groups who can blast out information about tons of bands. I write for a small zine - we get PR blasts all the time, often half a dozen band bios, album streams, and news from bands. What makes a band stand out from that and makes me actually listen to something from the amorphous mass of music thrown at me? Recognizing their name, knowing the band is on tour and I might see them, maybe a connection to another band, outstanding artwork, or a comparison to a band I love. But mostly, I don't listen to most of the PR blasts from agencies. I don't feel any sense of obligation to a PR agency to review every damn thing they send to me. I don't care if you're giving me advance tracks/free downloads, it barely feels different from spam when I get the weekly bunch of promos. Honestly, the ones that do stand out to me are the ones where the bands personally send an email. I've been slow on reviews lately, but I've been giving a few that stood out a quality chance, quite a few listens each. One is from a band called Countress whose aesthetic interested me, and the other is for Nocturnal Abyss as I was interested in Noctir's music due to his writing. The 200 brutal death/grind albums from Blast Head's PR blaster? Sorry, couldn't care less after the first dozen were boring.
However, there are more prolific reviewing outlets who review everything sent to them by a PR professional. In my unprofessional opinion, these high-volume review mills hardly consider the music and have shitty opinions and writers.
Bottom line: Unless you're doing a lot of other work to get noticed, a PR agency isn't going to do much for you. If you are getting your name out there in many other ways, then it's good to work with one to make sure your press pack is in the hands of everyone who might be interested.