Because looking at things in close detail all the time usually results in self-convinced failure. Photography is a good example of this. As photographers, we know what it's like to fail over and over. And over. "My photos suck. I screwed up a $10,000 job. I keep on getting undercut by the competition. I brought back a bunch of garbage at the end of the day." It's important to pay attention to the details and to look at the big picture both at the same time.