Nexus Procurement Suite provides an organization with control over what artifacts are allowed into a repository from an external, proxied repository such as the Central Maven Repository. Such control can be a prerequisite for organizations unwilling or unable to trust the entire contents of an external public repository.
If an organization is developing mission critical code, they will likely want to subject every third party dependency to intense scrutiny and testing before making an artifact available to build a release or support a team of developers. In most Enterprise development environments, a developer can't just decide to add in a new dependency to Hibernate or to the Spring Framework on a whim; the decision to add dependencies to third-party libraries will need to be funnelled through an oversight process that relies on an architect or an administrator to promote artifacts to a certified release repository.
Another, more common experience is an organization which needs to proxy something like the Central Maven Repository but wants to limit access to specific versions of artifacts or prevent dependencies on everything contained under a specific group.
Some organizations are more amenable to trusting the contents of a remote, proxied repository like Central, but they also need the ability to block certain dependencies. Maybe you work on a team that needs to limit access to dependencies with a certain license, or maybe you just want to make sure no one uses a problematic version of Hibernate with a known bug?
The procurement suite is the tool that provides for both coarse and fine-grained control of the artifacts that can appear in a repository. For more information, read the Procurement Chapter in Repository Management with Nexus.