The Copyright Clearance Center (CCC) is a collective licensing service. That means they work with rightsholders to manage permissions on the rightsholders' behalf, and the CCC takes a commission on licensing agreements they facilitate. The CCC offers both transactional licenses (for a single use) and institutional annual licenses that cover many institutional use cases.
The goal with collective licensing organizations like this is to have one place to go to negotiate your permissions, rather than tracking down rightsholders and negotiating permission individually. This streamlines what could be a complicated permission process. On the other hand, that convenience means organizations like CCC have a lot of control over the permissions market which could limit choices for users.