The clipboard is an OS feature that resides basically onto the copy and paste operations and is typically used for short-term data storage and data transfer between apps.
Clipboard managers are created to provide their users with additional clipboard functionalities such as storing several items at a time (instead of a single item, as is the default.
These specialized applications may also provide access to the clipboard data even after the host program was closed.
Clipboard managers are designed to increase productivity and speed for those who need to copy / paste numerous items.