1. Motivation. Together you can motivate each other to get into shape. If one of you is feeling weak (not physically) the stronger one has to pull the weaker one along. This wouldn't be possible without a buddy.
2. A Guaranteed Spotter. Some exercises will require a spot or help from someone else. There isn't always someone around you that can help you and a friend is perfect for this. Since you'll be working out with them for more than one day, they'll know what you like and don't like and therefore give you a better spot.
3. New Perspective on Working Out. New exercises, routines and ideas are important in keeping your routine fresh and exciting. Two people can collaborate and change a workout better than one person can alone. If your friend wants to try something new don't be afraid to, it might turn into your new favorite exercise.
4. Can't Let a Friend Down. Some days its tough to wake up, get out and go exercise. Its a lot easier to say no to yourself than it is to someone else since you don't have to listen to another person complain to you.
5. Friendship. More important than making your workout better, hanging out with a friend for an hour each day to exercise will help build a strong and lasting friendship.