They have to publish promotional CD so stores and radio stations can play their new music legally and people get interesed on it
Promotional CDs usually contains the same or less amount of songs than the original CD they are promoting
They can be like an exact copy of the original CD but with a text printed on the CD: not for sale, for promotional use only..
..or sometimes they don't have booklet and just a text with the artist and the songs printed on the CD