The flannel shirt is extremely popular when it comes to outdoor activities since ever.
Flannel is a soft woven fabric, of various fineness. Flannel was originally made from carded wool or worsted yarn, but is now often made from either wool, cotton, or synthetic fiber.
Which characteristics made (and make) flannel such an excellent choice for outdoor activities?
This question is clearly not opinion-based. One shouldn't answer with things like "Because it looks cool" or something but with rational facts.