Slug Barrels 101:
In the simplest term, a shotgun barrel is a smooth tube with a constriction, or choke, at the opening. It's purpose is to control the expansion of the shot string. The more the restriction, the tighter the string's pattern will remain. The use of chokes started a long time age.
The forum's grey-beards, like me, recall two major developments in shotgun technology: The plastic wad/shot cup, and the screw-in choke. When I was young, it was common for car's transmission to have only 3-speeds, these days my old car has a 6-speed. It's similar with shotguns, new ammo and screw-in chokes have added tremendously to the shotgun's flexibility.
Those shooting slugs don't care much about controlling the expansion of the shot pattern. After all, they are using their shotgun as a musket. Typically, slugs are used where rifles are prohibited for safety reasons (they have too much range) or you want to deliver a heavier hit than conventional rifles provide. It doesn't take a lot of thought to realize that if you dedicate a shotgun to shooting only slugs, then the choke doesn't really matter and some sort of sighting device (beyond a front bead) really helps. Now we have the birth of the slug gun (or the slug barrel).
The smooth bore slug barrel is typically fitted with some sort of musket/rifle sights. Since is not used for wing shooting, it's shorter than a bird barrel -- more like a rifle barrel length.
Now we come to the OP's question, what choke, if any, in a slug barrel and why? There are technical reasons and marketing reasons. Technically, in a perfect world, slug barrels would be choke-less and all slugs would be designed for choke-less barrels. But, this is not the case, slug manufacturers realize that not everyone who shoots slugs has a choke-less slug barrel and their ammo will be used in all sorts of shotguns. The answer is to make slugs from a soft metal and in a configuration that will allow them to safely pass the tightest choke. Because of this, many slugs do not perform their best in choke-less slug barrels. Typically, slugs provide better performance with just a pinch of choke. How big a pinch of choke depends of the specific slugs you are using.
What is the gun maker to do -- how much choke does he put in his slug barrel? If you are Remington or Winchester and you sell slug barrels and slugs, it's simple: You could make your slug barrels with just enough choke to perform best with the slugs you sell. If you are Mossberg and you don't sell slugs, then you have to guess what optimum minor constriction to provide. Another solution is to just use the same screw-in system you do with your wing shooting barrels and let the end user select his slug choke.
What's the shooter to do? For the casual slugster, you can get a barrel with a fixed "slug" choke get some slugs and go zero your sights before deer season. Next year you might get some different slugs to see if they are any better. Or, you can get a slug barrel with screw-in slugs chokes, buy several popular slugs and head to the range to see what combination slug/choke works best in your barrel. Remember, these are not wing shooting chokes like mod and full, they are usually very close in size and may be cyl, slug, skeet, imp cyl. Their differences will be measured in thousandths, not hundredths of an inch.
skinsman, I hope this cleared the muddy water a little.