Surely exercise is just about getting ripped, toned, slim or Instagram likes, isn't it? I'm not saying it's not about those things. But there's more to it, and some of that more is movement quality. Pretty much everything we do involves movement, from typing on a keyboard to bench pressing double bodyweight. So why care about how we move?
Pain and injury
In my experience, the biggest reason people start caring about movement quality is pain. The human body is amazing at adapting to what it regularly does. That's why professional athletes can hoist hundreds of kilos overhead or run a hundred miles. Unfortunately it's also why you adapt to poor movement habits: your body becomes very efficient at sitting behind a desk by creating tightness in your hips, shoulders and neck to hold the position cheaply. Smart, in a way.
The repercussion is that your body starts using compensatory patterns for fundamental movements like reaching overhead, walking and running. Strong, durable structures go underused while weak, delicate ones absorb strain they were never meant to. Small muscles get overworked (runners with angry ITBs), alignment is compromised (sore hips and knees), and movement options shrink. Follow that path far enough and you find backs going into spasm picking up a pen, ACLs rupturing in a gentle change of direction, and shoulders that can't shake hands without pain.
Performance
Still thinking you'll deal with it when the niggle gets loud enough? Poor movement quality is also making you slow and weak. Poor alignment bleeds force: the power you put into each step, lift or swing isn't transmitted where you want it, so you spend more effort for less output. You're a car with one wheel toed out.
Then there's resisting your own movement. That desk-adapted tightness has to be overcome before any movement has an effect, adding invisible resistance to every rep and every stride. Technically, this is neuromuscular efficiency: strength in one direction requires good recruitment of the agonist and appropriate relaxation of the antagonist. Unnecessary tightness is a tug of war against yourself, and it's entirely avoidable.
Keeping your basic abilities
Reason three is retaining the abilities every human should have: picking things up from the floor (your child, perhaps), sitting down and standing up, reaching the top shelf, walking and running without pain. Adaptation cuts both ways: it's use it or lose it. Look at which older people you know still move well; it's the ones who never stopped moving.
Conclusion
In our early years we learn movement skills unbelievably fast, progressing from rolling to crawling to squatting to running in a very short time. From there, whatever we do regularly shapes how we move for the rest of our lives. Whatever level of movement you aspire to, now or at 80, the way you move matters. Invest some attention in the skill of movement: avoid pain, perform better, and keep your independence. Future you will thank you.