Flannel as we know it originated in the Celtic regions of Ireland and Scotland. Although the fabric in question is indeed mostly referred to as 'flannel', its more proper descriptive term is 'tartan'.
Tartans were most useful in the distinction of different clans during clan wars. Different colours were used1 as well as various chequered patterns and stripes. The Celts, fully emphasising the tartan, understood the fundamentals of not giving a Scottish Kiss2 to a person of the same genealogy whilst in a brawl.
Flannel saw a resurgence in popularity with the Grunge Movement of the early to mid-'90s. Although the fashion in now largely passé, it still remains in some of the less trendy colleges.
Apart from the 'fashion' popularity of flannel to students of all ages, it has many useful properties. Flannel is largely used for warmth3 by fishermen, hunters, campers, and people with no apparent fashion sense in the late '90s. To the brave flannel-wearing souls, wear on: your determination is admirable.