It allows you to specify in more detail - someone who's not in any particular group, just has access, will be able to see access-locked entries but not custom-locked entries. Then you can choose to lock a particular entry to a sub-group of the people you've given access to, or a combination of access groups, or even just private. For instance, there might be things you want to lock to just family, or just people you don't know offline, or whatever.