How much protein and why?

My diary says I need 48 grams of protein. I actually have to make sure I get enough because I'd be content just eating vegetables all day. My question is why do I need 48 grams? Do I need more if I'm lifting weights or just working out more in general?

How much protein do I need each day?
There is no one-size-fits-all answer to that question, and research on the topic is still emerging. The Institute of Medicine recommends that adults get a minimum of 0.8 grams of protein for every kilogram of body weight per day—that's about 58 grams for a 160 pound adult. In the U.S., adults get an average of 15 percent of their calories from protein; for a person who requires a 2,000-calorie-per-day-diet, that's about 75 grams of protein. In healthy people, increasing protein intake to 20 to 25 percent of calories can reduce the risk of heart disease, if the extra protein replaces refined carbohydrates, such as white bread, white rice, or sugary drinks. Higher protein diets can also be beneficial for weight loss, in conjunction with a reduced calorie diet, although long-term evidence of their effectiveness is wanting.

For people in good health, consuming 20 to 25 percent of calories from protein won't harm the kidneys. For people with diabetes or early-stage kidney disease, however, the American Diabetes Association recommends limiting protein intake to 0.8 to 1.0 gram of protein per kilogram of body weight (roughly 10 percent of energy intake), since this may help improve kidney function; in later stage kidney disease, sticking to the 0.8 grams per kilogram minimum is advisable. Consult a doctor or a registered dietitian for individualized protein recommendations.