Satisfaction
The feeling of physically building something with your own two hands cannot be described. When you build something that is lived in my animals that produce your food, it is incredibly satisfying on a very primal level. Not only that, your chicken coop will stay standing for generations to come. You'll be able to take pride in your creation.
Unlimited Eggs
Personally, I love eggs. They can be used in a multitude of dishes, from breakfast omelets to hard boiled snacks. Even if you don't eat eggs very often, this aspect will mean the chicken coop you build will pay itself off. Not to mention, eggs make great gifts. Sell them at work or give them to friends and families to say thanks for a favor they might have done for you.
Design
Let your creative side go wild. Design or customize a luxury mansion for your chickens, or set them up in a minimalist apartment. It's completely up to you. When building a coop yourself, you can adjust the plans to fit the space you have perfectly. You can pick a design that you love, not something prepackaged without any love put into it.
For a complete step-by-step guide on how to easily build a chicken coop, the best guide available is "Building A Chicken Coop" by Ken Keene.
You can find my full review of it here.