Time is a Commodity
Time is an important resource and is often wasted because it is taken for granted. If we fail to set a timeline for a task, project, or goal, we will likely lose a sense of urgency to complete it as time goes by. Not setting an achievable timeline for completion is a common reason for an unmet goal.
Timelines are often given to us by people who hold us accountable. Teachers and bosses will give a due date and we often procrastinate until just before that due date. The sense of urgency is internal and keeps us accountable.
If we set a goal without a timeline, we will apply that same tendency to procrastinate until we forget about it or until a catastrophic event happens that brings the sense of urgency to the front of our minds.