They need to know how a piece reacts with the environment and with other materials. This field grew out of restoration, and you need to know the science and the craft of conservation. They handle basic preservation issues that involve artifacts, storage and handling , and maintaining records. Depending on the institution, collections managers are the link between the curator and the registrar. The curators are the art historians.
Registrars handle tracking an artwork's documentation.
They deal with the legal end of the business of art, including insurance, lenders, donors, etc. Typically, most registrars are not physically involved with the artwork. Of course, museums are key, but there is also a need for collections managers in historical societies, corporations, private collectors, and individuals. For example, on the corporate front, movie studios archive their costumes and therefore need people who can handle those needs. Collections Managers often have an art history or museology background.