Unity templates tend to be either learning tools or showcases for new features. The Runner template is the first template we’ve created where the thought process was, “This needs to be shippable and help our developers get from beginning to end.”
The Runner template is a performant starting point for anyone looking to rapidly prototype or ship a runner game. It comes with core game mechanics and essential features, including character controls, a track to run on, gates, collectables, etc. This means you can spend less time building basic subsystems and more time focusing on what makes your game fun and unique. The template is streamlined, since mobile games need to have a lower file size and take minimal CPU.
This template is not built for large AAA projects with intricate code – rather, it’s meant to be modified and personalized by all developers. For example, there are no assets included – everything is greyboxed so you can add your own content. Look at it as a starting point that you can build on.
Source: Unity Technologies Blog