While adding a collaborator as an editor means you can work on a session together, there are some actions which only the session owner can do. The key actions are:
Adding or deleting a day from your session
Deleting the whole session
Extracting a day from the session or merging it with another session
It’s always the session owner’s user rights which determine how a session can be shared. A session must have an owner but this cannot be transferred to another user.