The advantage of using a tooth positioner as a retainer is :
The use of a tooth positioner rather than final settling archwires has two advantages: 

It allows the fixed appliance to be removed somewhat more quickly than otherwise would have been the case (i.e., some finishing that could have been done with the final archwires can be left to the positioner)
It serves not only to reposition the teeth but also to massage the gingiva, which is almost always at least slightly inflamed and swollen after comprehensive orthodontic treatment. The gingival stimulation provided by a positioner is an excellent way to promote a rapid return to normal gingival contours.

As a general rule, a tooth positioner in a cooperative patient will produce any changes it is capable of within 2-3 weeks. Final (post-treatment) records and retainer impressions can be taken 2 or 3 weeks after the positioner is placed. Beyond that time, if the positioner is continued, it is serving as a retainer rather than a finishing device-and positioners, as a rule, are not good retainers.
Contemporary orthodontics- proffit 4th edition page 614
Final setting of occlusion
Used in uncooperative patients
Esthetics
Gingival margin inflamed during orthodontic treatment