While Sites is meant for organization of content, Sites itself won't control what content is available within the site. Sites are built off of groups, so you'd create a group for one division, and another group for a different division, and then you build the site around each group. The content available within each site is the content shared to each group. You can't have a site without a group.
So, you could rely solely on a group, but a group + Sites would give you that next level customization/divisional branding that may be appreciated.