At first they appear mutually redundant, but they way in which they work is
very different, and one might suit a given situation better than the other.
Share permissions apply to everything accessed via the share, but not when
the same files are accessed by a different route, for example via another
share, or directly from the machine's desktop. The share permissions remain
attached to the share itself, and no longer apply to files copied or moved
outside of the share.
Filesystem permissions always apply to the files and folders themselves, no
matter how those files are accessed. They support a much wider range of
options thatn share permissions, allowing more-specific control. Against that
they are more complex, and their effects are not always easy to predict.
Permissions attached to files or folders tend to remain attached to those
items even when they are moved elsewhere on the same disk-partition, which is
often not desirable.
Share permissions are best for shared workgroup-folders on a server.
Filesystem permissions tend to be used where individual users are granted
access, for example in 'Home Folders.'