(A) Beer. Many fine beers are not available with twist off lids.
(B) Not all bike riders are drinking at the bar or other place where there is ready access to bottle openers. Last time i pulled a bottle of Left Hand Milk Stout out of my bag while playing bicycle cops-n-robbers in a parking garage I was glad I had an opener on the bike.
(C) Cool kids add their own custom bottle openers, not just off the shelf stuff...
