The vertically dovetailed lumps were/are a marketing gimmick, a way to say that the 21 has "something that others don't". This is one of the areas where Winchester played up the "strength" bit. The truth is that, even if the breeches were simply flat and soft-soldered together, they'd be just as strong as with the dovetail. With chopper lump barrels (like the 21) the method of holding them together (soft solder, braze, super glue, etc.) actually has little to do with the "strength" of the action. Even if the dovetail were properly fitted, there would be no increase in strength, it would merely make the solder redundant.
The left and right barrels are made of the same material, dovetailing them together does not result in some new, stronger material (Winchester marketing BS notwithstanding).