If you take a package of deli meat and remove the slab, place it on bread and add your condiments in never tastes as good as when you take the slab and "fluff" up each piece as you lay it on your sandwich.
This deli meat you speak of is generally made of small scraps of mechanically-separated meat that are essentially "glued" together into a solid mass by enzymes that partially break down the tissue. This processed "meat brick" doesn't really have the same texture as an intact muscle tissue, which has individual muscle fibers aligned along a "grain" that makes it pleasantly chewy. Folding/layering/rolling slices of processed meat gives it more of a texture and chew, and tricks you into thinking you're eating an actual piece of animal muscle instead of meat-flavored jelly.
Similar to @uncle brad's comment, I would consider that it's related to the air:
A larger amount of air is trapped close to the ham, and hence is scented by it. This means that the ham-scented air is released into your mouth as you bite, which then escapes through your nose, increasing your perception of the ham (as taste is largely composed of smell).
I'm now actually quite hungry.
I think that Stuart has almost the right idea. Rather than trapping air close to the ham, and letting aromas diffuse through that, though, I think that it's simply a matter of surface area.
When you smell something, air flows over the surface area, and picks up the volatile compounds that form the aroma - those are what you smell, and clearly they don't have to have been trapped next to it to get a strong scent. When you eat something, you've got the smelling part going on, plus the food on your tongue - and surface area could easily help there too, letting more of the food come into contact with your taste buds as it's moved around in your mouth.
The best analogy, I think, is citrus zest. Clearly you can smell a whole lot more from finely shaved zest than you can from a hunk of peel, and this is true even without any air being trapped next to it. If you need convincing, just give some zest repeated sniffs, or carefully blow on it to remove trapped air, then smell it.
The increased surface area and air also allow the fat to warm up a little and give more flavor, so you're not eating stacked meat with still 'jellied' fat in the middle.
To fold ingredients to expose them to more air is a well known technique in cooking to ENHANCE flavoring.
The more air that infuses each bite, the better. Oxidation to food is essential to good flavour. So yes! When you make a sandwich, fold the meat to create air channels, if you have or can slice your cheese very thinly, then do the same as well.
If you make a sandwich with three pieces of meat, do not just stack them one upon the other; this is "truck driver" mentality. instead, layer them to enhance the flavour value. This is a well known technique among chefs and gastronomes alike.