When you want to read something, how do you make your choice? When you go into a library, where do you go first? When you go to the movies, what do you want to see?
When we read for recreation, we choose what to read based on the mood we're in. For sentiment, we choose romance. For stimulation, we choose adventure. For fun, we choose humor. For puzzling, we choose mystery. Every story has a theme, and we want to feel it.
Yes, there are subgenres and shades of emotion. Some adventure stories turn into horror. Some humor turns into romance. In the end, though, we get what we came for. If we don't get that satisfaction, that catharsis, we feel cheated.
At its most basic, genre boils down to tragedy and comedy. Will things turn out well for the characters, or do some of them have to die? Will we feel happy or sad when the curtain goes down?
Genre is about evoking an emotion. An abandoned house, for instance, triggers dread and not joy, while a brightly lit garden triggers joy and not dread. Writers use certain tropes to condition the reader, to influence the reader's emotions. Twisting or inverting tropes doesn't invalidate them, it only changes the perspective on them. Tropes have power. That's what makes them tropes.
When we read for criticism, we analyze. We look at themes (among other things). In a short story, there may be only one emotion. In longer works, the emotion may shift or alternate. As a basic story element, theme is as much a part of literature as plot or character. Genre helps to establish the theme, so it is important to analysis, albeit in a secondary role.