After-school activities take time. Participating means you will have less time for homework, less time to check your email and less time to watch TV. This is not necessarily a bad thing, though. Time management and prioritizing are important adult skills, and by adding an activity or two to your schedule, you can learn these skills by applying them on a regular basis.
School can be tough sometimes, leaving many children feeling as if they aren't good at anything, either academically or socially. Because you choose after-school activities based on your interests, the odds are that they will not make you feel this way. If you aren't good at something, you will at least be surrounded by people who are interested in the same things as you, which will give you social acceptance. Either way, a well-chosen after-school activity can be an excellent self-esteem booster.
After-school activities can also help teach a variety of life skills, in addition to time management. A cooking class, for example, teaches key skills necessary for feeding yourself and living on your own or in shared accommodations. The same goes for a driving class, a speech-making club such as Toastmasters and a variety of other after-school activities.
Sports are an excellent after-school activity because they promote physical fitness. No matter how chubby or skinny you are, physical activity is key to your general well-being. When you play a sport a few times a week, your physical fitness will be continually tested and improved. This will both keep you fit now and establish a firm foundation for your future physical fitness.