Currently, if you set up content.tree_root.location_id for a siteaccess that belongs to admin_group nothing changes when you access an AdminUI using that siteaccess. The expected behavior would be that when browsing/managing content, only content under content.tree_root.location_id would be visible. This is a useful feature for multisite setup.
The documentation already mentions that feature: https://doc.ezplatform.com/en/latest/guide/multisite/#introduction :

Each site will have its own content root, at a lower level than the default one (Location ID 2). You can use one global back office for all sites, or a separate back office for each site.

I am aware that location_ids.content_structure setting also exists (introduced in EZP-28624). However, it only determines the Content linked by the top menu. Content Objects outside the tree are still accessible (for example by breadcrumbs).