In a nutshell, mCOA is designed to augment the US Post Offices's NCOA (National Change of Address) service. As Vinay pointed out on his post, this is a nice addition to the NCOA service to help you clean up your mailing lists.
Cleaning your data should be something you do with relative frequency. If you snail-mail a high volume of mail, combining NCOA with mCOA might make a lot of sense. Talk to your mailhouse about this service.