Join Date: Aug 2008
Location: Fort Worth, TX
Bikes: 2006 Specialized Ruby Pro aka "Rhubarb" / and a backup road bike
The numbers encourage riders to register and pay for goods & services (rest/food stops, post-ride meal, SAG support, traffic control) received during the event.
Sometimes they help event photogs match riders to photos.
There may be requirements where the number must be worn (eg: races); sometimes it's fine to just zip-tie them onto the bicycle handlebars.
The numbers generally are not annoying to the rider when safety-pinned to the jersey pockets.
Some people attach them to a running belt that can go over a jacket only worn part of the ride.
Some people keep the numbers as souveniers just like some skiers keep lift tickets. Ever seen the jackets made out of recycled TyVek numbers? Fun.