Shared Lines can either be for voicemails or outgoing calls.
For example, if your business has multiple numbers, you can use a shared line to choose who you are calling 'As' by pressing a button on your handset. You can set members who have access to the group, and they will get a button on their handset to make a call from the shared line. This can be done per handset to multiple shared lines with the same number can be used.
OR
If a department has a group voicemail, you set the shared line to have members who can have access. There will be a button associated with the Mailbox on each members' handset.