One of the most important parts of being a professional (in any field) is staying up-to-date with the literature and technical/hard skills required to thrive in your field.
Librarians, like all professionals, should continue to both hone their current skills and develop and learn new ones.
Additionally, library managers and supervisors may wish to use online professional development to train new support staff and para-professionals or to cross train seasoned staff.
Here are some great online (and free) professional development sites for librarians and archivists.
Wyoming State Library Training Calendar
(up-to-date calendar of professional development training opportunities)
Image credit to Boston Public Library