It's not a question of serving a purpose. If you're looking for a "purpose" behind a costume component that might be your first issue. It's strictly an aesthetic choice. The red waist breaks up the blue in a way that is uniform and visually pleasing. If the end result is something a bit weird and goofy looking, that's OK. It's a superhero costume, and for a character like Superman a little cheesiness isn't inherently a bad thing. You take the trunks away, and you're effectively left with a naked man painted blue - which I would argue is just as weird and distracting to look at (if not more so). The belt does little to offset that visual.
Snyder was able to get away with it somewhat because he chose a darker shade of blue and added a bunch of seams and ribbing to the midsection to keep the eyes engaged. If he opted for a lighter shade of blue and had a plain midsection like the manip posted, I wager it wouldn't have worked nearly as well. I don't think it's an accident that Snyder decided against a proper belt and instead went for a gold band with a separate "buckle." It gives the illusion of a belt, but it doesn't clean-break the blue. So you're not left with the "blue shirt + matching blue pants" look.
Just thought I'd share my perspective. You're of course entitled to your opinion. I hope if the suit does have trunks that you'll be able to enjoy it for what it's worth. For my part, Snyder is the closest I've ever been able to get to accepting a trunks-less suit. Every other design attempt I've ever seen (especially in the comics) simply do not beat the original aesthetic. I'd go one step further as to say the all blue suit with the red belt is one of the fugliest things I've ever seen. The blue and red clash in a way that just doesn't work. The blue suit + yellow belt + red trunks is a perfect balance of all three primary colors.