Jiu Jitsu has become the #1 martial art in self defense because most physical confrontations end up with people grabbing each other and/or ending up on the ground. In these situations it is imperative to know how to use leverage to move the other person around so that you can control them, allowing you to stay safe, especially if they are bigger or stronger than you.
Mixed Martial Arts is a combination of three unique skill sets. Fighting on your feet (striking), taking someone down, defending the takedown, and clinch work or grappling on your feet (wrestling), and grappling, striking on the ground, and submission fighting (BJJ). MMA training, in and of itself, primarily focuses on combining skills you have already developed in other areas so it is imperative to be proficient in these areas before training for MMA fights.
BJJ is excellent for people looking to improve their fitness levels. Classes will involve learning how to move another persons weight around while they are resisting at varying levels. This forces your body to activate accessory muscles that are not commonly used in daily life, even if you are a well conditioned athlete of another sport. Another aspect of Jiu Jitsu beneficial to fitness is that it is also a complex & dynamic puzzle. Participants are mentally engaged in learning and solving the puzzle while the workout is happening, so it is much easier for them to push themselves physically for longer periods of time. So what can you expect from working out in BJJ? If motivation to workout is your issue you will have a much easier time getting into shape, you will become more toned, you will lose fat, and over a short period of time you will have much more energy.