It's hard to say, really.
Its about the aggression I guess. Controlled aggression. Rebellious and honest. As against pretentious, easy to do, and attention seeking forms of music. As against the hypocrisy of the formulas used by soceity to decide good and bad. As against God and everything that means. As against life, and what death means. Much in the lines of heavy metal and its philosophy, but darker. The music is definitely better than everything else out there. The skills of the musicians involved - second to none.
Well, the growls are there for a reason. They express things which words by themselves can't express well. Mostly sad things. Sometimes angry things. Sometimes brutal things. Mostly a combination.
The point is, the growling vocals are to be considered an instrument by itself, without considering the literal content. The tone of the growls, the depth in there. Having said that, the lyrics in death metal are extremely insightful. Well, extreme and insightful.
Especially Doom metal, a branch of death metal. Doom metal lyrics are sad. Very sad. They talk about things like loneliness, lost love and regrets, about death and the meaninglessness of life. The tempo is slow. And heavy. Like bass heavy, but melodic because the tempo is slow. The growling vocals talking about solitude, combined with the slow, building guitar and bass riffs, with drums setting the tone. It touches me in intangible places.