I have used case-assignment rules to assign cases automatically to the right user/queue based on certain conditions. 

Recently I was going through Trailhead content on omni-channel and various routing mechanisms (skill based, queue based). Following statement from trailhead confuses me : "Omni-Channel always assigns cases to the right support agents, and customers are happy with customer service. ".

If we already have case-assignment rules in place, why do we need omni-channel queue based routing? How are the two things different?