Homeostasis is the maintenance of a constant internal environment. The conditions it controls include body temperature, blood glucose concentration and the water content of the blood. These conditions are disturbed by internal changes, such as exercise raising body temperature or a meal raising blood glucose, and by external changes, such as hot or cold weather. Each is corrected by negative feedback: a receptor detects the change, a coordination centre processes the information, and an effector brings the condition back to normal.
It matters because cells work properly only in the right conditions. Enzymes control the reactions in cells and they work best at their optimum temperature, which is about 37 °C in humans. This is why thermoregulation matters. If the body is too cold, enzyme activity slows and the reactions are too slow. If the body is too hot, enzymes become denatured, because the shape of the active site changes and the substrate no longer fits, so the reaction stops.
Osmoregulation is the control of the water content of the blood. Animal cells have no cell wall, so they cannot resist changes in water. If the blood is too dilute, water enters cells by osmosis and the cells swell and may burst. If the blood is too concentrated, water leaves the cells by osmosis and they shrink and cannot work properly.