The Goods

 Trends come and go.
Often, what is popular one day may be obsolete the next.
Adidas used to be the dominant player in athletic gear.
Companies like Converse, Tiger and Puma (Formed after a dispute between the brothers who owned Adidas) were always nipping at the heals of Adidas in search of market share.

 When Phil Knight started Nike, he never imagined that he would be in a position to buy the entire Adidas company. (Nike now owns Converse and Adidas now owns Reebok)
Nike is now the dominant player in athletic gear.

But there is always another angle.
Large behemoths are too entrenched in success to be nimble enough to capitalize on emerging markets.
Companies like Under Armour (And now, to some extent, Adidas) take advantage of a counterculture not beholden to old power structures to create their own empires.
Sure, they learn the game from the big boys but they bring their own swagger, their own history, their own identity to create a new more competitive game.