Ham and tomato is the ham sandwich answered with moisture rather than acid, and the tomato is both the point and the problem. Cooked ham on buttered bread is salty and dry, and a slice of raw tomato brings the one thing that filling most lacks: a cool, soft, faintly sweet wetness that loosens every bite. That is what makes the pairing work. It is also what makes it the most structurally precarious of the sharpened-ham sandwiches, because a sliced tomato does not sit quietly. It bleeds. The defining fact of this sandwich is that its virtue and its failure mode are the same ingredient, and the whole craft is the management of a component that wants to turn the bread to pulp before it is eaten.
The craft is keeping the tomato's water out of the crumb. The slices are cut and, ideally, laid out and lightly salted so they shed some liquid before they ever reach the bread, and they go in against the ham rather than directly on the slice so the meat takes the early bleed. Butter spread firmly to the edges is structural here above everything else: it is the waterproof layer between a wet filling and a soft crumb, and a ham and tomato sandwich made without it goes soggy within the hour, which is precisely why it wants to be assembled close to when it is eaten rather than left in a tin. The ham is laid in enough thickness to hold its salt against the bland sweetness of the tomato, the bread stays soft and plain so it yields to the slices rather than crushing them, and the seasoning the tomato wants, a little salt and pepper, is what turns it from watery to bright.
The variations are the other answers to the bare ham sandwich and a couple of close cousins. Ham and pickle and ham and piccalilli reach for sweet-sour or hot-mustard sharpness instead of moisture; ham salad keeps the tomato but surrounds it with lettuce and cucumber so the wetness is shared out. Tomato with cheese rather than ham is the same wet-slice logic met by a different protein. Each deserves its own article rather than being crowded in here.