Well, some like the aggressive vocals. I love screaming and growling, as long as it isn't over the top. Here's an example of a song with much growling.
Maybe it's the vocals contrast to the melody, but the raspy tone brings a nice balance to the whole song. If it was all clean, the song would sound too light of tone, and it would ruin the mood. That's why I hate extreme death metal such as Cannibal Corpse. There is no balance between light tone and dark tone mooding, it's just dark with no sort of contrast. Don't get me wrong, there are some bands that do it good, like Adversary and possibly Slipknot.
Besides, can we explain why we like anything? We just do. I enjoy growling. Some enjoy clean. I enjoy the smell of gas, some find it disgusting. I listen to tons of music. I listen to mostly metal, but I also listen to country, rockabilly, psychobilly, electronic, rap, and so on.