I can think of several reasons: 1) discounts / promotions / contests and other forms of bribery 2) upcoming products you want to hear about as soon as they're

I can think of several reasons: 1) discounts / promotions / contests and other forms of bribery 2) upcoming products you want to hear about as soon as they're ready 3) products you genuinely love and want your friends to know about 4) companies you support for ideological reasons (they support some charity/cause you like) 5) social signaling / status (mark yourself as an Apple customer, or a Coach bag owner, or whatever.) 6) the brand puts out genuinely entertaining advertising (some people really like the Old Spice "smell like a man" commercials)