when it comes to cartooning, you are inevitably simplifying the real world down and removing details. It's an intrinsic aspect of comics that it simply can't be as complex and nuanced as the real world we see with our eyes.
With that in mind, exaggeration and abstraction become massively important tools for a comic artist. You can't capture every tiny detail of real life (which your eyes pick up automatically), so you are forced to find another way to represent the world around you, often by intensifying things that are normally more subtle in real life, or by showing it through some form of symbology that communicates a larger idea (think sweat drops, motion lines, impact effects, etc.)
When it comes to human expression, the brow is possibly the single most important facial feature when it comes to communicating mood. In the real world, humans have literal dozens of tiny muscles that can move and shift our faces around in a bajillion different ways, and our eyes evolved naturally to perceive and interpret all the tiny little variations of those muscles and the changes they make to the face.
Unless you are drawing in perfect photorealism, you will inevitably lose some of the incredibly intricate detail that is inherent to human faces and human expressions, and therefore need to exaggerate the aspects of it that you still have.
Thus, the highly expressive and communicative eyebrow being visible when they otherwise wouldn't be is an incredibly common and useful technique for making sure emotions read clearly to the viewer even in the simplified artwork of a comic.
TL;DR, eyebrows super important for reading expressions. Usually very useful to have them visible even if it's unrealistic.