The Common Blue Violet is actually quite common. Some years my yard is full of them, often in big clumps. Some years, last year actually, they were almost non-existent. I’m sure it has to do with the amount of rain. Last year was a very weird weather year.
The young leaves and flowers are edible. There aren’t many calories, but the flowers add a cute touch to a salad. In fact, some people candy them by brushing with egg whites and covering with sugar. Seems like a lot of trouble to me.