In Cabinet Mode you design a single piece of furniture. In Project Mode you create the environment (normally the room layout) of your project. You are then able to quickly position, move and modify individuals units within it.
Project Mode is great for 3D presentations and assembly plans, and to check your project is workable in situ. It also allows you to build more complex units in smaller parts, so they are easier to design and manufacture.
Note: please watch the Quick Design Toolbar videos on the Getting Started page first. These cover many of the basic commands in Project Mode.