You can train in karate for any of your own reasons. Some common reasons people may have for joining the karate club are:
Why take karate instead of a different martial art? If you are looking for effective self-defense, karate is a very direct, linear, and straightforward method of fighting, using practical basic techniques. In any martial art, as a student learns increasingly complex maneuvers, they become more and more difficult to apply in real life. Not that other styles are less effective, they just may require an extreme level of skill to realistically apply the techniques practiced. In karate, very basic and straightforward techniques are practiced until they are second nature, and are easy to apply in stressful situations.
Karate, as a "hard" martial art, is also generally a more rigorous physical exercise than most "soft" martial art styles, if you are looking for a workout. Karate is also taught in a balanced fashion, with equal time spent on arm and leg techniques, as opposed to styles focused on just punching, or just kicking.