Basically because the consumer considers that there is an (economic) «added value» that the brand provides to his «role» within the group to which he belongs («belonging group»). The choice of that especial brand generates in the consumer the feeling that he becomes part of a different social group to which he aspires to belong. This second group (called «reference group«) is the source form which the consumer takes attitudes, behaviors, styles, trends, lifestyles and relationship ways.
Every successful brand invites the consumer to buy it on a promise to bring him (or integrate him) into their most cherished «reference groups». These considerations are based on the deeper social and cultural roots of human being and the man tendency to move to higher «stages» inside the society which he belongs to. At this point «belonging groups» and «reference groups» are steps in constant motion.
Invite your consumers to be part of these groups and your brand will be really successful.