When you share documents, you grant access privileges to the users, so they can open the document and maybe work on them (if relevant privileges have been given to them). It does not mean that the user will for sure open the document and work on it. Collaboration, on the other hand, is a type of active sharing where the user actually refers to the shared document and (if relevant privileges have been given to them) modifies it. This is usually done when the document is in the preparatory or revision stages; e.g. a policy document being revisited by a team once every year to update it with any changes that may be recommended by this team. After every member of the team has reviewed the document, they can meet up and discuss the policy updates. If any changes are needed, these can be made and the document approved and then published. The new version of the policy would then come into force, replacing the earlier version.