Guests in your organization are typically; customers/clients, contractors, vendors who need to be accessible but they themselves need to have limited access.
In line with giving limited access; guests will now see the Projects, Discussions, Team sections in the sidebar only if they are part of any such entity.
Here is an example of a guest viewing very limited options on the sidebar.
If you include a guest in a Discussion/Team; then the discussion/team would start appearing in the sidebar for the guest.
Michelle was included in the team 'TeamZ'; which now starts appearing in the sidebar under Teams.