Having a plan when you head to the shop is the only reliable way to manage your time. I know that some of us relish in the spontaneity of building without a plan, but there’s just something so many ways to get distracted if you don’t know what part your working on next.
Yes, I could jot down some dimensions on a piece of paper of how I’d go about building a rocking chair for my kids, but there are so many things to keep in mind while building furniture that incorporates joinery. Are you accounting for your tenon length? Is your stock the correct thickness for the appropriate mortise? And then there are simple battles with proportion. The width of the slats on these rocking chairs could be decided upon in the shop, but dialing in their dimension ahead of time, based on a full drawing, will save you countless hours of debate and trial and error.
In my mind, spending a few dollars on a plan ahead of time is well worth the price if I can avoid spending my precious time in the shop scratching my head or throwing out a part that has a mortise in the wrong location.