You would only see a difference if you implement a role group.  If you create a role group, then it will add the name of the group to the drop down list.  If you select all roles, you get all roles available, if you select Global Roles you get all roles that apply to the entire site, if you select a role group then you only get the roles that are in that group. Each Role Group can have many roles that are only available to that group.  There may be a more technical explanation but that is how the two choices work. 

You have roles specifically, Admin, Host, registered users and a couple of others that apply to every page on the entire site regardless of what you do.  You can also build roles on your own that aren't assigned that apply to the entire site.  At least for me, I wouldn't use role groups if I was going to have them working on the entire site, so for my purposes I use role groups to break off roles that only edit certain sections.  It probably isn't the best use for role groups but it satisfies a need.  You could use your role group for the entire site just as well.  Hope that helps.