I'm sure it's all part of the "rock star" look. I also suspect they're prescription glasses, for those times he doesn't want to wear contacts. The guy is almost 64...he's gotta have SOME sort of eyewear, at this point.
A lot of stars have the habit of wearing the sunglasses all the time. They may be just for the looks, including gene simmons. There are many colors and designs of the sunglasses for you to choose from. You could find that wearing sunglasses is really cool and fashionable. You could also wear in this way to show your fashionable way.
He wear sunglasses for cool for sure, and the black sunglasses are his preference. As an artist, Gene Simmons can be called as the most unique one, keep wearing his black sunglasses can make his funs love him more. Because they can feel the emotion expressed by the black sunglasses.
Most of the celebrity love sunglasses very much, and lots of them have their classic version with sunglasses.
From what I can see, you must be quite into that singer huh? As a matter of fact, sometimes he would like to wear sunglasses when off the stage, so as to keep his privacy and try to be mysterious. However, when he has got a show to perform, he would simply take those sunglasses off, and you could observe his face decorated with some cosmetics, which is quite strange but awesome.