Would you enter a race if you didn't know where the finish line was? Of course not. You need a finish line to know when you're done running and when you can claim the prize. You might be the most amazing coder, solid worker, or incredible designer, but if you're going to get a win on your project, you need a clear idea of how to make that happen. If you don't figure it out ahead of time, those traits will quickly disappear and you'll have no cheering throngs to lift you up.
Effective design requires a finish line to promote motivation and fight mid-project fatigue. If you start a project, you need to know how long the race is— otherwise you'll just wander around aimlessly without ever getting the win.