Because the director of Twilight, Cathrine Hardwike, thought that it would be cool for the Cullens to have something that symbolises them. The guys wear the wrist band, Rosalie wears the pendant and Alice wears the choker.
The Cullen's Crest is a family crest that was in the movies. It wasn't in the books.
the Cullen crest
Rosalie wears the Cullen crest on a chain around her neck, while Alice wears it on a small purple ribbon around her neck, in choker form.
Like what is it? It's this: http://media.photobucket.com/image/cullen%20crest/JoeJonasLuver12345_2008/cullen-family-crest.jpg?o=11
The Cullen family Crest is made up of: The hand, The Lion, and the Trefoil. The hand = Faith and Sincerity The Lion = Strength and Ferocity The Trefoil = Perpetuity
It actually doesn't say anything about a culln crest in the books but thy added it into the movie to prove they are cullens and to have a part of their family with them i guess. It doesn't exactly specify.
Its a lion.
The wristband has the Cullen crest on it. All of the cullens wear this in some way. (the girls have necklaces)
the family crest( thats what my brother told me)
the symbols are called crests. A knights crest showed what family he was from because each family had a different family cres. A knight could also wear his kings crest to show what king he served
In Twilight, the Cullens are Edward's (Robert Pattinson ) family.
I don't know about clothes (seeing as there'd be a long list), but everyone of them wear something they like to call the 'Family Crest'. Bella, of course, doesn't wear one as she isn't one of the Cullens yet. Alice wears the crest as a choker as seen below... Rosalie wears one as a pendant necklace as seen below... Carlisle wears his as a ring as seen below... Esme wears hers as a bracelet as seen below... And Edward, Jasper and Emmett wear theirs as arm cuffs as seen below... Well, there is also a bracelet that Bella wears in the movie that people are kinda interested in. Look below... That's all I can find on that, so I hope that helped.