What is the benefits of having the heart enclosed in a sac that has pericardium?

Many. The pericardium isolates the heart from surrounding organs, prevents the heart from over expanding when blood volume increases, limits heart motion, and keeps a small amount of fluid around the heart that severs as a lubricant which reduces friction with surrounding tissue as the heart beats.
RV strain. My personal opinion is that the pericardium serves to protect the heart inside the chest cavity but just as importantly, constrains the rv during acute overload (i.e. Sudden increase in venous return during exercise) which might cause the rv to fail.