1. Burn Fat. Weight training helps burn fat. It increases your metabolism rate by up to 15%. However, you may not notice a big change when you're stepping on the scale. Muscle weighs almost twice as much as fat. So replace that fat with lean muscle and don't worry too much about what the scale tells you.

3. Strength training also helps reduce bone deterioration and builds bone mass. In turn this prevents osteoporosis.
4. Balance and coordination will improve. So will your posture.
5. Boost your energy level by adding weight training to your work-out. Weight training boosts your level of endorphins and makes you feel great! It's also a fantastic anti-depressant.
6. Toned muscles are always nice to look at. Let's not forget the important aesthetic benefits of weight training. Be proud of your body and all the hard work you've put into it.
Always remember to start gradually. Don't lift too much weight when starting off.
Give your muscles a day of rest to recover.
Always stretch and warm up before resistance training.

Knowledge is power. Educate yourself on weight training. Find a routine that works for you. Learn which exercises target which muscles.
And finally try to keep your body guessing. I find that switching it up when it comes to weight training helps build muscle mass a lot faster. Don't just stick with one exercise for each muscle group. Keep your body guessing and it will in turn keep building muscle mass to cope with the next workout.