Training your dog to heel will keep him under your control when you are out on walks. Dogs who walk on leashes in municipalities that require a leash will need to be under your control to keep themselves safe, as well as the people and animals around them. Training 'heel' keeps your dog’s manners in check. If you hike with your dog or live in a more rural area and would like your dog to be off leash, heel is a great command to train because you are going to want your dog to be near you while you are walking in open areas. Your dog wants to please you, and staying by your side and walking with you using the heel command is the perfect command to show you respect.
Training heel with treats is more manageable than not using treats because the treats reward your dog for a job well done and encourage him to continue to learn new things and appease you. Heel also teaches your dog who is master; leader of the pack, so to speak. Teaching your dog to heel, whether on leash or off-leash, will be easier starting on a leash. The heel command will give you control over your dog's movements. Teaching heel with treats is easiest with dogs who are younger, but can be taught to older dogs as well. Just make sure you have a treat that is enticing enough to grab your dog's attention. Be sure your dog knows his basic obedience commands as well.
To train heel with treats, you are obviously going to need treats on hand. Keep a pocket full or a baggie of treats with you so you can train longer sessions and still have enough rewards to keep your dog motivated. You may want to find a long fence or a wall to put on the other side of your dog as you are teaching your dog to heel. And you will need to a leash to get training started, even if your goal is to have your dog off leash once he understands the command heel.