Updating and refreshing a Library both have to do with getting content into the Library. Updating a Library makes the Server check its folders and sub-folders for new or removed media.
If it finds new or unmatched media, it then scans it into the Library and fetches metadata for it.
Settings to control automatic or periodic Library updates can be found under feature.
: A "Refresh All" is really just a specific version of a Refresh in which the refresh is applied to an entire Library.A Library Update typically needs to happen every time you add or remove media files, and so it's useful to be automated.