Once you’ve seen a well executed hand-sewn button hole, anything made by a machine is just depressing. They’re miniature artworks and the skill required to make them well takes years to fully develop.
When apprentices are learning to sew buttonholes, they’re usually referred to as cockroaches or dead spiders (the buttonholes, not the tailors) because they typically look awful and stay awful for a long time, until fine motor skills develop and muscle memory improves, allowing nearly identical stitches to be made, so the buttonhole takes on its 3D shape and clean appearance.
With that said, as beautiful as they are, they’re useless. At least they’re no more useful than a utilitarian machine made buttonhole, which a machine can make in under 3 seconds.
Which is a drawn out way of getting to the point about what actually matters when it comes to hand sewn components of a suit.
Pick-stitching, hand-sewn button holes, embroidery, are all aesthetic improvements, but make no discernible improvement in the performance and wearability of a suit (or shoes). Where hand sewing improves how a suit performs is in giving softness and flexibility to critical areas, including the armhole, collar, chest and lapels.
Machine made stitches, even advanced modern machine made stitches are unable to match the softness of a stitch sewn by a human hand. Machines tend to pull and lock a thread tightly through rapid movements, which makes for a harder, stiffer feeling when a jacket or trousers are being worn. Conversely, a hand sewn stitch has more give and flexibility between stitches, allowing the cloth to move as it’s being worn, pulling less against the wearer and allowing the whole item (jacket/trousers) to move as well.
Looking at the internals of a hand made jacket, the herringbone stitches are evenly spaced, but can be adjusted in key places (spaced more closely together or further apart, as well as being pulled more taught or less so), to create a 3 dimensional product. This is most obvious around the lapels and collar. If you want the lapel to roll strongly, the stitches are made smaller and placed more closely together, which rolls the cloth back onto itself more noticeably. The same goes for a collar, where a tailor encourages the collar to sit firmly against the neck by narrowing the stitches, or relaxes it by widening them.
Machines can do this, too, but they lack the inherent feel and softness of the human hand, which lets the part of the suit behave as desired, but to do so with a great deal of forgiveness. Effectively, with a rigid stitch, there is no “give” except for what little flex is in the cloth. With a handmade stitch, the stitches themselves act as hundreds of tiny dampeners, allowing a seam to flex, before the cloth does. It doesn’t sound like much, but when you consider how much the human body moves, even in relatively mundane tasks, the culmination of those thousands of instances when you’re clothes work with you, rather than against you, ads up to a much more comfortable experience.
What you’ll often see on brand-name suits, including most of the well known brands at the high/luxury end of men’s suiting, is a machine made suit, which is partly hand finished. i.e. pick stitches along the lapels and seems and sometimes handmade button holes. The immediate impression is that the suit is hand-made, but that’s misleading (intentionally) because only the final details have been completed manually, which have zero bearing on how the suit feels when worn. Machines are also now able to replicate the imperfections of the human hand, creating a pick stitch that looks like it was done by a person, when it wasn’t. Fortunately, no machine has been able to replicate a well made hand sewn buttonhole, yet, so that’s something to be thankful for.
Not as importantly, but still usefully, a hand-sewn button (not buttonhole), when done well is better than anything which can be produced by machines. Silk thread is waxed and steamed (melting the wax into the thread, strengthening it) before the button is sewn on, leaving a small gap between the underside of the button and the cloth, so the thread can be wrapped tightly around the vertical threads, forming a stem which stands the button up like a soldier and makes it nearly indestructible. You’d rip the cloth apart before the button came off.
Hand sewing in shoes is most valuable in the welt (hand-welting as opposed to the machined goodyear welt). Just as with a suit, hand-welting a shoe makes for a softer shoe, right out of the box and beyond, as the tension is controlled by an individual, rather than a machine rapid-firing stitches into leather. This is also a major reason why hand-welted shoes are typically more expensive than machine welted shoes.
The debate about the value of hand stitching for aesthetic reasons is another one altogether. From a purely practical point of view, there are several reasons where hand-sewing is and isn’t important and it’s equally important not to get caught up in the hype of aesthetics over substance.