In short you could say that Unlined or Unstructured jackets are the most natural fitting and draping jackets.
Before we go into the benefits of an unstructured jacket, we need to do a short history lesson first. After WW2 the majority of men would wear a suit because they had to. Alongside that we have seen men who aspired to create their own success and businesses which led to different play fields with new dress codes and possibilities to express yourself with your personal style. Actually we could see them as the contemporary Dandy.
However, before the cultural revolution of the 1960's men used to wear suits because they had to and because they liked to. Oddly enough, we're now coming full circle and man men (spearheaded by the millennial generation) are now wearing suits because we love the way they make us feel, look, and act. While our fathers may never understand this drive, our grandfathers certainly did, and wore their suits and sport coats for the same reason.
More often than not this is seen on lighter, summer-weight fabrics. Part of that is because lighter weights of wool and materials like linen are more prone to wrinkle in the first place, and another reason is because removing the lining allows the jacket to wear much cooler. Natural fibers like linen, cotton, and wool can breathe but this breath ability is obstructed by the satin lining, making even light-weight jackets wear much warmer in the summer.
Thankfully though, casual jackets are appropriate throughout the year and many men are making unstructured jackets from heavier materials like tweed and winter-weight wools. These casual winter jackets wear just as comfortably as their summer counterparts but still retain a bit more heat and keep with the seasonal propriety of each fabric.
One of the only downsides to an unlined jacket is that they're near impossible to find. One of the benefits of a lining is that it can hide the ugly-but-effective stitches that go on inside a suit. An unstructured jacket can't do that same thing, so the makers are required to spend more time and effort with stitches that both do the job and look good at the same time. For most off-the-rack makers, the demand isn't worth the extra cost. This does not count for genuine tailors. This means you can come in for a visit and tell us - with any material you choose - whether or not you want to unstructured your jacket. There are no limitations and you have the complete freedom to build your sport coat or suit as casual or formal as you'd like.