Say you work in a matrix organization. Projects are broken down systematically in work packages. When a work package is ready to be scheduled, in Splitvice you launch a work item. It's now up to the team to get that work item done.
In this matrix organization, the team lead distributes the work in his team, often together with the team members. Now, say this assignment is poorly done, and the work is assigned to somebody with a lot of other work already on his plate? That would hurt your project timing, so while it might be the right choice, it's good to have this choice and the result transparent.