Although I think you're baiting, I'll bite.

I really enjoy eating, but if I'm caught up in something I don't always remember to do it. Sometimes I'll go hours past my regularly scheduled meal times. Then I'm sitting at my desk wondering (only for a short time) why my stomach hurts. Oh ya, food.

While this is a extreme example, the logic still applies to coding/writing or anything else. Sometimes the tasks you should do are side tracked by other tasks. And if you're deep in thought, you might not have the internal alarm to switch to something more of a priority. Hence external alarms.