I understand that food cooks differently in a double-boiler than when cooked "directly" - but don't know why that is.  How does utilizing a double-boiler change how food cooks? 
Use of a double boiler limits the maximum temperature the food being cooked can reach. The water in the lower part of the double boiler can only reach 100*C (212*F) before it boils. The food being cooked, therefore, can only reach a maximum temperature of 100*C (212*F). It can't go over that until all the water in the lower pot has boiled off. 