Because when I have my club ready and know my shot selection, my shoes are tied, the cart is parked and my clubs and balls are clean, I hydrate and refill my water bottle, I check my yardage and wind, update my scorecard, and the group ahead of us is still on the green for 10 more minutes (this goes on nearly every hole), I can always eat a granola bar or some trail mix.
Keep the blood sugar and electrolytes up, especially through 18 holes. Fitness and diet and hydration don't matter in the first 6 holes or so, but can easily make a 10 stroke difference on the back 9. This is why I usually beat my partners in matches; I keep my focus on the back 9 and go Tiger Woods on them while they start to lose focus and get sloppy. There's nothing like hitting a 300 yard drive on 18 to intimidate a tired opponent who's already lost ground. Plus if you get into trouble you can still be aggressive, and have your full arsenal while they are desperate for you to go down easily because they know there's nothing in the tank. This goes double if they take 90+ strokes to get around, taking all those shots can wear out the core and legs and force them to generate power in bad ways.
I don't like to eat a big fattening breakfast, maybe a piece of fruit or some yogurt and some coffee. Even then I'm forcing myself to eat. I have absolutely no appetite for about 2 hours after I get up. Then I eat most of my protein afterwards.