Research has shown that muscular people live longer and have a lower risk of all-cause mortality (heart attack, type 2 diabetes, etc.). As people have more and more lean mass, their chances of dying get lower and lower. This is why strength training is so important, especially as we age.
When you gain muscle, your resting metabolic rate goes up, meaning that your body is burning more calories at rest. That's worth mentioning because those are calories your body is burning that you don't have to move for. And the more calories your body burns at rest, the easier it is to maintain a calorie deficit.
Hope this makes sense. If you already know this, sorry for repeating the obvious!