what for
whether you have one or multiple sites, a CMS should allow you to manage and consolidate all your web operations on a single platform to streamline processes and provide significant savings on development and/or maintenance costs.
for whom
for those who don't have the time and resources to build and maintain a site, to begin with, or for development agencies who will hand over the end product and the management responsibilities to the end user.
which one
to answer this, you have firts to carefuly consider and preferably test a CMS features, its potential scalability, 3rd party integration options, weight up cost and, most of all align them with your own or business needs.