I have a 50mm F1.8, and now I have a 28-75mm F2.8. What reason would I need that extra little bit of aperture with no ability to zoom? This isn't a rhetorical question. I need to understand these things.
Well the operative word here, I think, is "need".
Apertures, or f/stops, when broken down by thirds, are recognized as being: f/1.8, f/2, f/2.2, f/2.4, f/2.8
So, going from f/1.8 to f2.8 equates to a little over one full-stop of additional light. Depth of Field t is also something that could be considered. Just how useful one additional stop IS for either reason (depth of field or the extra stop of "speed") is
One of Those Questions (by which I mean to say a matter of opinion and entirely subjective).
Also under consideration is that prime lenses are typically sharper than zooms. Notice I said, "typically" because there are exceptions, of course (aren't there always). Exactly how MUCH sharper, and whether or not that sharpness matters from a practical stand point, is also
One of Those Questions.
My personal opinion is that if you know what you're doing you can adapt. If I have a prime I zoom with my feet. If I have a zoom lens, maybe I use that feature, maybe I still zoom with my feet; it all depends on what I want and I don't whine I need a zoom when I have a prime, I adapt to the situation and take the best shot I can. I don't get overly involved in the "What if..." sort of scenarios that many people seem to relish. Life is full of missed shots... Always has been always will be. I do what I can with the lens I have and get on with Life. I don't worry about shots I can't get, I focus on the shots I can get and on making those shots as good as I possibly can.