If you are really good with CAD and have the program along with all the symbols you need to create landscapes and don't need to do 3D imaging, you could do it easily. Most of the rest of us are not proficient with CAD nor own a copy, so we rely on software written for the intention we need. The 3D imaging part of the program that is included in many of the software packages is an excellent presentation tool and will help you sell work by allowing your client to understand what you are trying to accomplish. If you know CAD, then you also know most people don't and are only confused by the typical 2D plan. I add color to my hand drawn plans and people are impressed, still confused, but they think it is pretty. When I show them a 3D image of their home with plants, hardscapes, structures and lighting they now have a visual reference. I also explain that the installation will NOT look exactly like the 3D image, but it is a representation of the goal we wish to achieve.
You need a good set of plans for the installation and materials take off, so no matter how you arrive at that goal, it all works. The key to good design is not how it is drawn, but how it is created. Is it sustainable, attractive, safe, environmentally sound, artistic, unique to the property owner and does it meet the needs of the client? How you get there is not as important as these considerations and more.