Strength Training Is..
Strength training is mobility training. You can do all of the stretching you want but if you are not accessing and strengthing those ranges of motion in your training then you are doing yourself a disservice. If you can't access a position under load, you are predisposed to injury.
Strength training is injury prevention training. A stronger, more durable muscle is going to not only give you a higher output for a longer period of time, it is going to be able to withstand the demands being placed on it. A stronger muscle protects the tendons, ligaments and connective tissue when making sharp cuts, decelerating and stopping on a dime. Weak muscles expose you to consistent small tweaks, sprains and soft tissue injuries. Stronger muscles make you more resilient and reduces your chances of injuries.
Strength training is longevity training. We don't become frail because of our age. We become frail because we become inactive, no longer putting our muscles to use, and surely not eating the proper amount of protein at this age either. If you never stop strength training in some capacity, the frailness can't set in. Strength training increases bone density, and muscles are what move bones. Elderly suffer from weak and inactive muscles and weakened bone structure. Strength training combats both of these things simultaneously.
Strength training is mental resilience training. Even those of us that love to train, don't always want to go train. But you will never regret getting a workout in, ever. Strength training is your personal testament with yourself that you can do and overcome hard things. Sometimes its nerve wrecking picking up some weight or trying a new weight you've never done before, no matter the age or experience level, but it shows yourself that you can do hard things. Pushing through hard workouts, doing the extra rep when you wanted to cut that set short, keeping your form strict when all you want to do is set the weight down and give up. Strength training is much more than just physically beneficial.
Strength training is sport specific training. It will build your speed, power and explosiveness. Again, a stronger, more durable muscle is going to not only give you a higher output for a longer period of time, it is going to be able to withstand the demands being placed on it. All athletes can benefit from getting stronger. If they are getting stronger in the weight room, I guarantee they will be seeing improvements in their chosen sport. Stronger muscles will allow you to 1. recruit more muscle fibers to produce more force (sprint faster, jump higher) and 2. protect our joints from the forces that are imposed on them like decelerating, planting their feet in the ground, changing direction and rotating. This benefits all athletes and will have transfer to their sport.
Strength training is healthcare. They can't sell you strength training to receive the level of profits that they can from medications, pills, shots, doctor and hospital visits, so they don't try to get you to make it a regular part of your life. Why doesn't your doctor prescribe a strength training program with daily walks and a whole, single ingredient food diet? This will literally solve a plethora of "problems" for a lot of people, reducing the trips taken to the doctor and required daily medication, therefore you are no longer a customer they can profit off of. Take control of your health. Strength training is one of the absolute best things you can do not only for yourself, but your loved ones and everyone else around you. Be an example, not a liability.