Traditional martial arts systems are the most common form of self-defense out there. While this is my favorite type of fighting style, it's not a good fit for everyone, so carefully weigh the pros and cons before you commit to this type of system.
Modern self-defense systems focus on survival and combat sports focus on winning a competition. Traditional martial arts systems, in contrast, focus on personal development and physical health. You will learn many techniques which will help you escape from an assailant or win a duel against a fellow martial artist. However, your chief goal is to overcome your shortcomings and refine your character through developing discipline, patience, humility, and other virtues. In other words, your goal is to defeat yourself. For this reason, the traditional martial arts are a lifelong pursuit. Some examples include karate, Kung Fu, Tae Kwon Do, and Japanese jiu-jitsu.
The chief benefit of the traditional martial arts is that they exercise your spirit, your heart and your mind in addition to your body. They also provide a very deep knowledge of their particular aspect of fighting. Essentially, they are the graduate-level courses of self-defense. In addition, they include many exercises which not only improve your health, but add years to your life. The most famous of these are the kata, which are basically yoga poses for warriors. Others include breathing control and meditation. These systems also teach you the traditions and history of the nations which they come from, so they are a fun way to gain insight into another culture.
The main drawback of the traditional martial arts is that they include many techniques which have been rendered obsolete by modern weaponry. You may know twenty different ways to disarm someone with a spear or a katana, but will that help you against a gangster who is packing heat? Additionally, many of the techniques have only been utilized effectively by the master who developed them. This will make them difficult to learn for a student who does not have that master's body type and personality, if not impossible. The combatives tend to be unnecessarily complex, and you must memorize far more than you could ever realistically use in order to progress in the system. Defenses against modern weapons are usually not taught until you are a first-degree black belt, which typically takes 3-4 years of consistent training to earn. You rarely practice drills for multiple opponents, if you do at all, and many of these drills are not practical unless you are a young, athletic man. Furthermore, most of the traditional martial arts only focus on one area of fighting, which can get you into serious trouble if your assailant takes you out of your element (i.e. you are a striker and they take you to the ground). In the United States, the most effective techniques (such as groin attacks and eye gouges) are typically removed from the system for fear of offending parents or to prevent lawsuits resulting from the actions of irresponsible students. If they are taught, they are generally withheld until you are an adult or a first-degree black belt. In other words, your six-year-old son or teenage daughter might be stuck using pressure-point strikes against a 200-lbs. sex offender because the instructor you picked made your sensitivities more important than your child's safety.
You should study this type of system if you are truly passionate about the martial arts and want to cultivate your character as well as your body. Ideally, you should already have a basic knowledge of self-defense and want something more fulfilling than one-upping a fellow warrior in the ring. That being said, if you are a professional fighter, the traditional martial arts will give you an edge over your opponents by improving your technique, focus, and endurance. You should also study the traditional martial arts if you want to teach self-defense for a living or if it will help you in your profession (i.e. you are a police officer or security guard). The traditional martial arts are highly beneficial for physical health as well, so they are perfect for you if you are a mature adult who wants to get back into shape and improve their quality of life. Not to mention that, if you are interested in the culture which they came from (for example, China or Japan), this is a cool way to learn more about their history and customs.