It's not common sense to most people, that's precisely why it gets recommended so much. It's very easy for you to say it is, perhaps because that's all you know. For some people you can get stuck in a sort of auto pilot, moving from acting on one thought to the next, instead of realizing they're just thoughts and you can let some go.
I'm also a bit skeptical that you actually are mindful of yourself. When you actually do it and realize how it works, it's pretty amazing. I certainly didn't think "well, that's just common sense". When you actually do it, start observing your thoughts and feelings as separate from you instead of consuming them and getting caught up in them, it does bring calm and a feeling of refreshment.
The best thing I use mindfulness for is stopping a destructive cyclical worry. As an example from the past, say I'm worried about a client not having paid his bill yet, I know they're having cash flow problems and I worry I exposed myself to too much risk. What can happen is that thought can spiral and keep popping into my head, intruding into my day. It might create a cascade of thoughts, what my bank balance is like, how might I get a partial payment, is it too early to phone, should I look for another client, what will I do with the existing work, should I have asked for better payments terms, etc., etc., etc. But the worst part is, I would have that exact same chain multiple times a day. It intruded over and over, until the client would pay (predictably) late again.
But none of that helped when it was firing, it's not even that late yet. It just made me stressed and stopped me from getting what I needed to get done that day, sometimes I couldn't just place it aside.
Using mindfulness, instead of consuming that thought and worrying about it, I can just look on it as a thought.