Sometimes things you do seem pointless, a waste of time and effort.
You end up losing focus, getting distracted or diverted, and ultimately ending up either not finishing the task or not meeting the expected standard.
This is where having a purpose helps.
Purpose helps you stay focused on a goal.
It makes you see a point in things you do. You no longer feel like wasting your time.
You no longer get diverted (unless you are an inherent procrastinator).