Well, off the top of my head, one thing that catchlights do is draw attention to the eyes which, generally speaking, are the "focus" of a portrait.
For example, I just grabbed a photo I took a while back of my daughter. I did no "retouching" of this for any skin stuff, so you could look at flaws. But, I find myself drawn to the eyes, and the little catchlights are a big part of that:
Here's another example, where there was daylight giving some "natural glow" to the eyes, but I enhanced it a bit by bouncing some "fill" toward the face which seems to attract a bit more attention to the eyes:
So: the two questions you could consider. First, do you want to draw attention to the eyes? And second, what would the eyes look like without the catchlight, and would the absence of the catchlight add or detract from the photo?
And, outside of portraiture and the use of a flash, the fact is that the eyes do naturally reflect light. Without a flash, the reflection can add to the image or the lack of it can be, well, lackluster. Or, in some scenarios, you may not want a reflection at all, so really it's up to you and the look you are after.