OK, theory is this. When you consume protein (meat), it gets partially digested in the stomach through the action of acid (HCl) and a protease called Pepsin. Hydrolysed and partially digested proteins then enter into small intestine where they further get degraded by the action of at-least three proteases called Trypsin, Chymotrypsin, and Carboxypeptidase. This results in the release of individual amino acids and very short peptides that then absorb into the blood and get processed in the liver.
Now meat is not just protein but minerals and vitamins too. While minerals can sustain cooking (heat), vitamins often get destroyed. For those who are interested in building muscles, vitamins are as important as are proteins. Though vitamins can be taken as supplements, why not together with dietary proteins? Secondly, there is no clear evidence that cooked meat provides more energy compared to semi-cooked or raw meat. Cooking does denature (partially digest) proteins but at the same time coagulates them too that renders proteins resistant to the action of proteases in stomach and small intestine. This would eventually result in less amino acid release per gram of meat. However, I am not advocating consumption of raw meat. One can strike a balance by consuming half done meat.
About boiled meat... this technique works best for tough, cartilaginous cuts but not for lean cuts. Body builders prefer to consume ground meat (again to expedite digestion process) as petties or meat balls any way, and thus would not gain much benefit from boiled meat.