It’s one thing to set a goal, work hard at achieving it and along the way you see incremental progress. That progress signals you’re on the right track. Your efforts are working, which inspires you to keep going. It’s why losing weight and accumulating wealth are among the dominant topics in personal development. You see the pounds on the scale, the dollars in the bank account. Just like functions in mathematics, for every input of effort, there is a measurable output. Do “X”, get “Y.”
It’s an entirely different thing when you have no idea if the steps you’re taking are actually working. Artists can toil away for years, for decades, without seeing any tangible progress or fruits for their efforts. Monetarily speaking, it will probably never work out.
Jack Lemmon (my all-time favorite actor) was once asked how he knew to stay the course during all his lean acting years.