Manual sharing grants access to one record at a time when organization-wide defaults, the role hierarchy, and sharing rules would open too much or cannot express the exception. A record owner or another permitted user shares that record with specific users, public groups, or roles so a coworker can collaborate, cover an absence, or join a special project without seeing the rest of the owner's records. Use it for one-off exceptions; use a sharing rule when the same access should apply automatically and at scale.