Strength training helps you:

Develop strong bones – By stressing your bones, strength training increases bone density and reduces the risk of osteoporosis.
Control your weight – As you gain muscle, your body begins to burn calories more efficiently. The more toned your muscles, the easier it is to control your weight.
Boost your stamina – As you get stronger, you won’t fatigue as easily. Building muscle also contributes to better balance, which can help you maintain independence as you age.
Manage chronic conditions – Strength training can reduce the signs and symptoms of many chronic conditions, including back pain, arthritis, obesity, heart disease and diabetes.
Sharpen your focus – Some research suggests that regular strength training helps improve attention for older adults.
-source: Mayo Clinic