the bloodstrem has two "cycles", one: heart - lung - heart and then following heart - the body - heart. the second part is much larger and it requires more strength to get the blood circulating, since the whole body needs to be supplied. the left side of the heart is responsible for this, while the right side only needs to pump the blood to the lung and back. since the two different chamers have different requirements in terms of strength, they are built differently and therefor asymetrical...
the condition where the heart is leaning to the right side is situs inversus... all organs are mirrored
Edit: the evolutionary advantage ist we get better blood supply because the heart is optimized for the pumping capacity needed and therefor we perfom better at tasks that are physicaly demanding.