Ham and coleslaw leads on the slaw, because the slaw is the part that changes a plain ham sandwich into this one. Cooked ham on buttered bread is salty, lean, and slightly dry on its own. Creamy coleslaw, shredded cabbage and carrot bound in a sweet, vinegared mayonnaise, brings three things the ham lacks at once: moisture, a raw crunch, and a sweet-sour edge. The defining character of this build is that the wet, sweet, crunchy mass and the cool firm meat are doing opposite jobs, and the contrast between them is the whole point. The coleslaw is not a garnish stuffed in beside the ham; it is the half of the sandwich that makes the other half interesting.
The craft is containing a filling that is, by design, wet. Coleslaw weeps dressing as it sits, and a generous spoonful straight onto the crumb soaks through to a slick within minutes. The usual defences are a slaw bound tight rather than loose and sloppy, drained if it has gone watery, and laid against the ham so the meat acts as a partial barrier between the dressing and at least one face of the bread. Butter to the edges on both slices is the real waterproofing, sealing the crumb against the vinegar and the mayonnaise. The bread is soft and plain because the slaw already supplies all the texture and acid the sandwich needs, and a chewy or assertive loaf would only fight a filling that is meant to read as cool and fresh against the salt of the ham.
The variations mostly swap the slaw or the cut. A cheese coleslaw slaw turns it richer and saltier; a beetroot or apple slaw shifts the sweet note and stains the bread; a chunkier homemade slaw brings more bite and less drip. Ham with piccalilli or pickle reaches for the same sweet-sour relief from a jar instead of a bowl, and ham and salad leaves the binder out altogether. Each of those is its own sandwich and deserves its own article rather than being crowded in here.