Crewcut, apparently you don't see the value in landscape design, and that will translate to you potential clients. People pay big money for designs and all that goes with them, because they are created by professionals with experience in horticulture, hardscape, structures and design. The drawing is a small part of the design package. When a design is created correctly, you are taking the clients thoughts and desires, and with your talent, skill, knowledge and site conditions, creating a landscape that will be functional, sustainable and beautiful for them and their home.
You would not and in most states cannot, build a home without a plan. You will be much more successful in business with a plan, and avoid a great deal of unneeded stress. But if you choose to make it up as you go along, your work will reflect that. A well thought out landscape design with plant list and materials list will not only make your life easier, but will giver your client the opportunity to understand what it is you are trying to accomplish. You can easily increase your gross income with artistically created landscape designs, because the client can see and understand the project better and are willing to spend the money to have something they want installed, rather than something you think might be cool. When the cost of a properly installed landscape is between 10%-20% of the value of a home and you are working on a home that would sell from $250k on up, most homeowners want to know where their money is going, before they write the first check.
How can you get the job, unless you have a plan? I can assure you, many folks make a very comfortable living as landscape designers or architects from just drawing the plans and selling them to clients. Many smart landscape installers use independent designers to create landscapes for their clients, rather than have a person on staff that eats into profits everyday. Other designers will work with a group of installers to implement their designs, and some sell the design to the client with the intention of the client doing the install themselves. It stops people from picking up a plant at home depot with no clue and bringing it home without a notion as to where or how it should be planted. If you cannot afford or think a complete design is needed, a consultation may be all that is needed to fine tune or tweak a landscape at an hourly rate. If your clients are only going to spend a few hundred dollars on a landscape, you are right, no need for a designer, but when they are spending tens of thousands and more, everybody needs to be on the same page, and the few thousand spent on plans and pretty pictures will definitely be worth it.