Increasing muscle mass will enable your body to burn more fat during cardio. Lowering body fat equals less chance of heart disease, not to mention several other potential health risks. A weightlifting program will produce more capillaries (blood flow) throughout your muscles, making it easier to transport oxygen and other nutrients throughout your body. Weightlifting will make you physically stronger. A well defined weightlifting body is more attractive than an out of shape body. Following a weightlifting program makes you more rugged, for example if your fell down your body would be a lot less brittle than that of a thin person, and have less chance of being injured. Weightlifting gains will give you a self confidence that you have never felt before.