In the real world, unlike the unlimited reality of the 3D computer world, you are constrained by many variables: time, money, materials, gravity. These and other realities place limitations on your design vision. Only your imagination is unconstrained and unlimited. These projects give you constraints or limitations and ask you to find solutions to a design problem within these limitations and constraints. The tools you use are strictly defined. The purpose of the design must be met.
You'll notice that the directions for these projects are very brief. Except for a few parameters regarding what materials and tools maybe used, the rest is up to you!