There are many ways that other artists throughout history have gone about creating their paintings and preparing for the compositions. Some artists will plan out every detail of a composition in small miniature versions of the larger painting, in full color studies. Their reasons are that it saves on time, paint, and mistakes later on. Other artists such as Velázquez, prefer to jump in per say, with little or no planning out, and they cherish the discovery, creating in the moment, problem solving of the process. Velázquez would add on sections of new canvas, sewing them on with a cloth support behind the linen, as his compositional needs demanded.
I would liken it to driving automatic vs manual, or better yet, driving with a GPS to an unknown location, versus getting in the car, rolling the windows down, hand out of the window to feel the air currents and driving down unknown roads to see whats on the other side of that hill. Either way gets you to where you are wanting to go, just depends on your focus and intent of the journey.