Have you ever had to drag a heavy suitcase through the airport? Or move a box of books? The heavier your load is, the harder it is to move. The further you have to go, the more tired you get. In an extreme case, if your load were too heavy, you could potentially injure yourself moving it. There are some loads you simply can’t safely lift.
Our knee joints are much like a person carrying a heavy load. For every 10 pounds of weight we carry, the knee carries 15 to 20 pounds of pressure. Fortunately, our knees are designed to carry the weight of our bodies, so this doesn’t usually cause a problem.