1. You get the gym [mostly] all to yourself.
The number of people that go to the gym in the morning is much smaller than those that go at other times. Less people means more available machines, and more available machines means an easier fitness circuit. Less people to share machines with, and less people to watch you trying to awkwardly maneuver the pull-up machine.
2. The camaraderie. 
The early birds are hardcore individuals, up at hours of the day when most people are still asleep. It's an exclusive group, and its members are proud to be in it. When you go to the gym at the same time and cross paths with the same people throughout the week, you develop a silent camaraderie. A shared respect for their shared commitment to early gymming.
3. For the sense of accomplishment.
Working out at the beginning of the day leaves you feeling fresh and revitalized the rest of the day. Not only does it boost your metabolism, but it gives you endorphins that leave you feeling good. When you face all the challenges that the day has in store, you can face them with a little more bounce in your step – and endorphins.
4. You free up your day.
Now that you've had your exercise for the day, you don't have to worry about it until the next day. You can check something off of your to-do list, and focus on other more pressing matters. It isn't like you'd be doing anything else during that time – it's too early to be doing anything but sleep. So once you've exercised, you don't have to worry about fitting it into your schedule the rest of the day.
5. Practicing self-discipline.
As a morning person, I can enjoy getting up early to go to the gym. However, there are definitely those days when I would much rather stay in bed and sleep than go to the gym. But on those days that I push through my discomfort, I enter and leave the gym with a great feeling of accomplishment. And every time that I push through, it gets easier for me to push through the next time.
