Sometimes your day gets the better of you. No clear goals. Without a specific sense of purpose, it's impossible to effectively manage and organize your priorities. Lack of priorities. The best to-do list is useless if it hasn't been prioritized.
Most people naturally work on the easy or fun things first.
No daily plan. Beginning your day without a plan of action is a formula for spending all day doing the wrong things. It invites anyone and everyone to interrupt your activities. You will passively allow unwelcome intrusions because you'll have no way to defend yourself. After all, tomorrow is another day.