"Set-in" vs "Flat" sleeve sewing -- is there really a difference? When sewing for costuming/cosplay, most of the patterns I tend to turn to have a &quot;flat&quot; style sleeve construction; that is, the front and back pieces of the garment are joined at the shoulder, then the sleeve is attached while both parts are still able to be laid flat, before closing both the sleeve and sides in one long, single seam.
However, the pattern I'm currently working with calls for a &quot;set-in&quot; style of sleeve construction--the side seam of the garment is closed up first, as is the sleeve seam, and then the finished sleeve is inserted into the armscye and eased into place before being sewn.
This raised a question for me of what the difference between the styles actually is; I find it easier to sew a flat sleeve, so is there a reason I should do the more time-consuming set-in sleeve instead? A search of some blogs and forums resulted in mixed responses; some said &quot;there's no difference, do whichever you like,&quot; while others espoused the superiority of one or the other technique without any reasoning to back up their assertion.
So my question is simply this: What is the real advantage (or disadvantage) of a set-in sleeve vs a flat sleeve, if any?
To be absolutely honest, the difference is marginal in most (but not all) cases.
You probably won't even notice any difference in:

short sleeves
wide or baggy garments (like a tunic or pirate blouse)
garments where the shoulder seam extends far down the shoulder (like a kimono or most casual menswear)
garments sewn from stretchy fabric

Puffy sleeves that are gathered at the top of the shoulder should always be set-in sleeves because that allows you to place the gathers where they need to be. In theory you could gather the (open) sleeve first and still use the flat method, but I honestly find it easier to use the set-in method in this case.
In tight fitting garments that don't stretch (like formal business shirts or coats) a flat-sewn sleeve might be too tight to accommodate the shoulder. The shoulder seam usually sits on top of the shoulder and the sleeve points slightly down at an angle. But the human body doesn't have angles like that, there is still the round of the shoulder that has to fit in there:

The picture is a tiny bit exaggerated to make the difference visible. (Original image source)
The blue line represents the flat-sewn sleeve. The fabric forms a straight tube without any ease, but the round top of the shoulder somehow has to fit in there. Bulky people will notice the difference more than skinny ones.
The green line represents the set-in sleeve. It was cut wider than the armscye on purpose, then was slightly gathered on top to shorten the seamline to the same length as the armscye, before it was set in. Since the sleeve cap is a curve, some parts of it are cut on the bias and can compensate the gathering without puckering. The result is not a straight tube, but one that has a bulge at the top to accommodate the shoulder.
Here are some tutorials for set-in sleeves that clearly show the too-wide sleeve being gathered into the armscye:

Tutorial: Setting In Sleeves
Set in Sleeve : Perfect way to attach sleeves