That might be a question you ask yourself when you have a project in mind. I'm not bashing the idea of buying plans. There's a lot of requests about buying plans, so I thought having a discussion about "to buy, or not" might help. I'd like to also include the freebe plans that are available.
Most custom woodworking and cabinet shops can't buy explicit plans for projects that walk in the door. From my own experience I can say it can be pretty scary. Even after all these years it's a challenge to take on a project and be responsible for gettin' it right. So, how is it done?
My method is to just look at the idea, and figure out what steps I have to go through to make it look like the idea. I may do many rough pencil sketches until I get very close. I may present those sketches for approval.
Among the rough sketches, I'll do a rough explosion drawing to show the joinery and general details. Then I take the sketch and draw it to scale on a drafting table (that's right, a drafting table). I'll draw a floor plan, plan views, elevations, sections, and detail drawings. I know I've posted this before, but I feel getting specific in the "to buy plans or not" thinking process might make trying your own hand at the planning stage would broaden your scope as a woodworker.
At first for some it might be easier to buy plans just to see how someone else relates how to make or build something. Plans are generally set forth in a typical format. A materials and cutlist may also be included. Seeing how that's done could be helpful. Once you get the procedure for drawing up a set of plans, you will see how to improve it by just doing the work as the plan indicates, and your work progresses through to completion. You might find areas of improvement in details, or maybe drawing a joint from a different angle.
For a large project like a kitchen, I use a method like I found as a kid putting model cars and airplanes together. I make several copies of the pages of the plans and mark right on a page in either a letter or a number for each cabinet, the doors as they lay on the cabinets, drawer fronts, drawers, shelves, and every panel that makes up the entire job. Those numbers or letters then get put on each piece as they are cut. I wind up with several sheets, each may be for a specific part. For example, one sheet may be just an elevation showing all the doors on the cabinets, with their corresponding number or letter. I will also have a cutout sheet with many rectangles representing 4x8 sheets and draw on each rectangle the cutout layout for each numbered or lettered item.
What does that do? Well, it tells me how to cut up a sheet, which sizes first and in which order, which allows me to maximize material and minimize waste. It also will tell me if I'm missing anything, by just looking at the parts and their markings and comparing them to the drawing. It's a fool proof system that works great. It sounds like a lot of paperwork, but, a project with a lot of parts can be confusing when cabinet panels can be close in size to some doors or shelves.
There are other advantages. It keeps parts separated and will keep parts that need certain machining readily available. This all may sound confusing, but developing your own methods takes its course and will become a habit that just happens. For other projects, the paperwork may not be as involved, but the design end of planning a project into plans become easier.
Doing your own plans will bring you closer to the work that is needed to be done, as you will be thinking of the variables in making the project from just an idea or rough sketch. You may find yourself understanding joinery to a greater degree in just planning out which joint to use where. Personally, I find as much fun in the planning stages as I do in the fabrication. It's not that difficult...you'll see.
Besides, if you get in a jam, there's always help on the forum.