Chastity belts were originally a method of ensuring that ladies could not have sex and that their "purity" was assured. In BDSM play, chastity belts are still popular for similar reasons.
Particularly in long term relationships, chastity belts may be used for ensuring that the slave does not stray from her Master or Mistress. Just as importantly, it allows the Master / Mistress to restrict the slave and deny her the chance to masturbate and pleasure herself. With the chastity belt in place the slave can still please her owner and she can be stimulated, but she is unable to achieve the orgasm or release she will quickly desire.
Chastity belts for BDSM play will often have dildos or vibrators attached. This gives the slave constant erotic stimulation, and she is unable to stop it. Chastity belts are greatly enjoyed by Masters and Mistresses because of the control it gives over their slave, without causing pain.