This question is a funny one because it depends on the personal experience, the size of the boat project and amount of obsessive compulsiveness of the boat builder to truly appreciate the question and provide an answer.
Sanding the bare plywood before applying epoxy will theoretically improve the finish. You are correct. If that is the only finish being applied. So if you are making a bright finished dinghy or lightweight boat with only the chine and transom seams being taped and glued you'll see a difference. If the boat is anything bigger, see below:
However, when constructing a boat most of the plywood will then be covered with fiberglass cloth which when wetted out is transparent, but will lay over the now sanded plywood and require some type of surface treatment as well because of its lack of UV resistance, it's weave and inevitable bumps, lumps, wrinkles, loose fibers, dust, insects trapped in the goo, etc. That is why most hulls of any size, using fairly coarse heavy fiberglass cloth requires an additional step called "fairing the hull" to obtain the smoothness of finish that you refer to.
Even with multiple fill coats of epoxy and many hours of sanding with progressively finer abrasives, many top coat finishes or primers require a certain amount of "tooth" to adhere mechanically if they are not capable of a chemical bond. Thus you'll often see surface prep done with no finer than 120 or 220 in most cases.
If you want a superfine bright finish over a wood epoxy fiberglass composite sandwich then Varnish is the way to go. After many rounds of sanding and multiple coats of Varnish with obsessive attention to detail during application and good lighting to reveal "holidays" or spots that didn't get covered, you'll have a baby butt smooth and glossy mirror like finish that you'll love and admirers will go Gaga over. And when it inevitably gets scratched, will be fairly easy to touch up with another layer of varnish.
The most frustrating part of building your own boat is when it seems to be nearly done, there is a lot of "finish" work that is between you and the end. Most of the finish work takes the form of sanding. And then sanding some more. That is why you hear a lot of discussion about workboat type finish vs. yacht finish. Most of the difference is amount of surface prep and attention to detail before and during the final topcoat/paint application. It is also what causes some projects to take a lot longer than initially estimated, because it is your pride and joy and every imperfection, drip, ripple, etc. must be corrected. It is why some people, refinish their boat after it is done because they weren't satisfied with the amount of "whatever" when it was done. It is also, what sadly causes some projects to be assembled, but never finished.
The truth of the matter is, very few or no one else will notice the little things that nag you about the finish. Unless stored in a museum, even after a modest amount of careful use, your boat will be worn, bird pooped on, spider stained, mineral and black stains from water, and scratched beyond comprehension. Obsessive attention to detail on the bottom, is especially unnecessary if your boat is larger than a car topper, stored on a trailer, stored on a lift, or have its bottom painted and kept in the water.
The bigger the boat project, the more complex the rigging and inevitably more surface area required to be finished, the more time involved, and you can see it only gets worse. It is amazing what slapping a coat of paint can do to improve your mood when it comes to working on a boat.
I look forward on my next boat to the liberal use of non skid paint on the top sides to provide an attractive, durable finish that can be applied without a ton of sanding. Perhaps even spray the whole thing with Rhino liner and be done. Heavy, butt ugly, but indestructibly done.
Good luck with your boat building projects.