The magic part for me, is what LOOKS decorative, is actually functional and contributes to a long wearing, warm garment that should hold up with daily wear in the cold season... and made and shaped by hand. I think wool has to be one of the most adaptive fabrics for many kinds of applications. Then, that hand-stitched looping pattern to ease the band into the crown... sigh. Genius.
Another example for me was finding out that the Norse (and Saxons) used "broken twill" for weaving garments, which also had a purpose. I always found diamonds and chevrons that didn't "meet" a little "off" until I learned it wasn't a fashion choice.
Modern testing (probably by archeologists and re-enactors) has shown that broken twill protects more from a knife or projectile wound than pointed twills or plain weaves.
The Norse used many plain weaves (tabby), and there are a few examples of pointed twills. But in a more violent age, having more excellent protection of the broken twills for outer garments, especially for men, makes sense. So decorative braids (litter braids in a modern sense) are used inside a garment to strengthen the seams and decorative trim to make them even more robust.
Years ago, I blindly followed some instructions to knit a cap from Rudd's book on the history of knitting, as there was no picture. This cap was one of a number found dumped in the mud in the late 1400s (think Conquistadors) and was worn by foot soldiers of the time.
The result looked exactly like a conquistador helmet, only in wool instead of metal. The speculation is they were either worn under the metal helmets to avoid chaffing and/or they were worn by foot soldiers who wanted hats that looked like the metal helmets. Both answers are probably accurate.