3D modelling allows you to communicate your current and new ideas more effectively to your end client than 2D CAD.
Improved visualisation as to what the end product will actually look like is more easily translated, especially with the addition of detailed rendering, which isn't an option with 2D CAD.
3D assemblies improve collision detection between components, especially within larger assemblies where there are a lot of components and high quantities of components. This also allows you to get the correct amount of quantities, saving time in the future.
3D modelling allows you to create drawings direct from the model, along with bills of materials in a fast effective manner. This will also aid in eliminating drafting errors .
3D design enables a faster product design process which can be up to 45% quicker on average.
Revisions in 3D CAD are much easier and more economical than 2D drafting.
Each component cant be separately edited without changing the overall design and can also be reverted back to a previous design if necessary.
3D modelling can be used to generate images for illustration manuals. Exploded and fully rendered views are particularly good for this.
Increased productivity. 2D CAD requires a lot more evaluation to ensure all the separate views are correct to enable accuracy when it come to manufacturing/assembling the product(s).
3D CAD allows you to isolate, test and analyse individual components without it altering the rest of the design.