Firstly: Yes. You can do yourself significant harm by being stressed and tense all the time. Just ask anyone with high blood pressure or low back pain that required invasive surgery that had a low success rate!
I don’t think I have time to list all of it. It would be a novel. To start, I’m just going to list one physical benefit, and save other physical reasons and any emotional/stress related reasons for later, if you’re still interested.
There are tons of reasons why a muscle might be getting told by the central nervous system to continue a low-grade firing response (ei. tension). Imagine it like a cramp, but a minor one. The problem with your body creating a tense muscle 24/7, is that the body gets tired of maintaining that extra tension for long periods of time, yet is being told that it must.
The body is very efficient. It takes more nutrients to maintain that shorted, tense state than a relaxed one… so your body will take a short cut that requires less nutrients/resources. Microscopic strands of scar tissue will form in the muscle to keep it shortened like that, so the body doesn’t have to actively maintain it anymore. Like scaffolding, slings, or bracing.
The problem with that is that its commonly uncomfortable (ex: bad posture and range of motion), and severely limits flexibility. In instances where you need flexibility, instead of stretching, you will get tearing. You will be more prone to injury down the line. That’s not EVEN getting into the kind of chemical byproduct buildup that comes from this kind of scenario… that’s a post for another time.
A massage is, in essence, gently (or sometimes not so gently, depending on what the client wants or needs) breaking down those microscopic straps of scar tissue to allow appropriate blood flow and flexibility back into the muscle, and can also interrupt the signal from the central nervous system to keep firing. You hit the reset switch.
Sorry this is so long, and yet… this really is just the tip of the iceburg.