Our bodies are designed to move all day, and in a number of different ways. Our hunter gatherer ancestors would complete everyday activities over and over, with no help from machines, no soft couches to sit down on, and no modern conveniences such as bench tops, or a tap to turn for water.
We can get through the day without moving a whole lot, and not expend a great deal of energy. Things like cars, offices and office chairs, and the internet help us to perform our work while being super sedentary. Even sleeping, something we do a third of our lifetime, would not have had any plush cushioning to enjoy. Living in luxury has meant our bodies are subject to reduced load on muscles, tendons, joints and so on. Never before have our bodies had to perform so little.
It is temping to think short term about contributing factors to injury when we have pain that has come from seemingly no where. Your knee, for instance, might have recently become sore performing fairly mundane daily tasks such as sitting down in a low chair or walking up stairs, and there does not seem to be any one specific cause or reason you can think of that may have caused it. Was it from doing that walk on the weekend? Was it from squatting to pick up your daughter three weeks ago? Or was it from that knee injury when I took up bike riding three years ago? It's helpful in this context to think about more than what you have been doing for the last three days, three months, or even three years for that matter that as to what may be contributing to this new pain. What do your last three decades look like? Have you been performing consistent, frequent squatting, bending, lifting, carrying heavy things, digging, grinding, walking, running, jumping, day in and day out? I know I havn't. I have spent hours on end sitting on soft couches, lying on comfortable beds, using a dishwasher and washing machine to clean, preparing my food standing at a bench top that is a very convenient height. Sure I have run, played sport and gone to the gym, but this pales in comparison to what was demanded of the body before we made our lives far more comfortable.
All of this sounds obvious, but not engaging in enough "general movement" throughout the day is super problematic over a long period of time. From things like osteoarthritis, osteoporosis, cardiac disease, diabetes, etc etc. The advice to counter these problems is to perform regular exercise. But what does it mean to exercise? Exercise is a made up term to describe a specific set of movement patterns with a defined structure - eg you perform 30 minutes of running at moderate intensity, or do 3 sets of 10 weight lifting activities. It differs to "physical activity" (also a made up term that is now needed to explain normal human daily movement because of how lazy we have become) in that exercise is a subcategory of physical activity that is planned, structured and purposeful for the reason of improving physical fitness. Humans have been intuitively moving and "exercising" long before we had a strict definitions in place.
Exercise is beneficial to us when done appropriately for the reasons that we do live in 2019 and not 38,000 BC, and we need to place an appropriate physical stress on our body because I do not have to hunt for my food.
But it is the variety of movement throughout the week, both exercise AND the day to day tasks that help build a robust and resilient body free of pain and injury. Further if you are injured, it helps to take a zoomed out birds eye view of your problem in the context of your movement patterns (not how much you have or have not exercised) since you were crawling on the floor, through childhood to where you are now.