Protein is an essential element of any diet. Every cell in your body is using the amino acids that make up protein to function. It is a key part of life, and necessary for survival.
The goal of weight training is to breakdown your muscle, and rebuild it stronger than it was before. When you are lifting really heavy weight, you are tearing the fibers in your muscle. In order for those fibers to repair and re-grow, you need to consume protein after your workout. Consistent weight training sessions let your body know that "hey, I need more muscle in order to lift weight", and it will want to respond by building bigger muscle in order to meet the training demands you are putting on yourself. Muscle is mostly made up of water and protein and cannot re-grow without it. Failure to eat protein within three hours of your workout will result in little to no gains. After you finish training, the body will go into recovery mode and work hard to rebuild, but it cannot do this unless you feed it the amino acids that it requires. You also need protein to maintain your current muscle mass, so even if you are not lifting weights protein stills plays an important role in maintaining healthy body functions. If the body cannot get adequate protein from food, then it will respond by breaking down it's own muscle mass to help cells continue to do their work and keep you alive.
If you have spent hours weight training but are not making gains, you may need to look at how much protein you get everyday. Most likely you are simply not consuming enough to be able to repair, rebuild and re-grow your muscles.
Feel Full Longer
Protein takes longer to digest than carbs or fats, moving slowly through the gastrointestinal
Burn More Calories
The breakdown of protein within the body also burns more calories than the breakdown of carbohydrates or fat. Anytime you eat something, you body will burn calories simply digesting it. Eating a diet rich in protein can boost your daily calorie burn. If you have two people (assuming they are eating at the same time of day and have the exact same genetic makeup) who both eat 1,500 calories a day, except one of them is consuming more protein than the other, the person who is consuming more protein will have a higher metabolism.
Slow Rise in Blood Sugar
Protein also slows down the conversion of carbohydrates into glucose in your blood-stream, keeping your blood sugar levels stable. Remember that protein takes longer to digest, so when you pair it with carbohydrates you have less chance of your blood sugar rising like it normally would when you consume only carbs. A sudden spike in your blood sugar triggers the pancreas to release insulin, and a sudden increase in insulin greatly affects your body's ability to burn fat.