Device detection is integrated into site preference handling. Your initial site preference is set to the current device type making the request. You can then select to view the other site and future requests will respect that preference. Essentially it is a way to override the view as determined by the device detection.
Technically, site switching is not redirection in the way you think of redirection in Spring MVC. Site switching supports three different URL strategies that give the appearance of redirecting to a different site. In reality all of your views are stored within the same application. The mobile and normal URLs are mapped to the same site. You then use device detection to conditionally display view data for mobile, tablet, or normal devices.
It is certainly possible to accomplish this with just Spring MVC. In fact, Spring Mobile is built on top of Spring MVC, using the features of Spring MVC. The point (and goal) is that if Spring Mobile provides functionality you can use, then you can just add a few lines of configuration and not have to worry about it yourself. And if it doesn't quite fit your needs, then we would appreciate hearing about how we can improve it.
Spring Mobile Projects Lead