Did you know if you don't use a muscle it will get smaller or "atrophy"? Atrophy is a decrease in muscle mass. It can be caused by disease or accidents, but for most of us it is caused by lack of use. Take my sweet Grandmother for example. She decided to spend the last years of her life sitting in her front room chair. Because she chose to not move much, it became difficult for her muscles to even help her get up. Some of that is from the aging process. Some of that was simply from disuse.
Thankfully, most of us can reverse muscle atrophy in most cases through use. Having enough muscle supporting your joints can be critical to preventing injuries. Think of your knee for example. If you walk your dog every day, the muscles around your knee will be stronger than they would be if you didn't. Then, when you step off a porch onto a rock and your knee goes sidewards, those muscles are supporting that joint and your ligaments don't get torn or stretched too far as easily. (Ligaments join bone to bone at joints.)
Help muscles help you by making them strong!