building muscles is good because....

Building muscle IS good. Muscle is more dense then fat so weighing the same amount with more muscle makes you look better. Also, muscle burns calories, even at rest, and increases your overall metabolism. Another thing is that the more muscle you have the more effective any exercise will be because all muscles in a targeted area increase fuel burn. Lastly while repairing itself (to increase muscle you actually tear muscle tissue and the body repairs it stronger. thus the soreness after a workout) muscles continue to burn fuel, this can last hours after the workout ends.
There are some things to remember when weight training: 1) there are limits to how much muscle the body can put on so continuing mass building for extended periods will have diminishing returns.
2) Muscle requires more energy, and different kinds to help build it, foods high in protien are good for this (but read up on it as there are many different things your body needs to build muscles).
3) big muscles with weak bones can make for stress injuries and breaks, so make sure you eat all the things you need to keep your bones healthy, Adults will loose bone density as they age, women in particular, so make sure you keep up.
Other then that, Go for it. Weight training is great for you! If you really want to do it right though, buy a session or two with a trainer, because I see soooo many people using horrible form at the gym. Much is wasted sets for them, and they are putting their body in a bad situation.