Why would putting a lid on a pot which is cooking reduce heat loss?

Water will boil faster with the pan lid on. Keeping the pan lid on traps some of the heat and steam in the pan which would otherwise escape, thereby increasing the temperature, and bringing the water to a boil more quickly. To keep a full pot from boiling over, be sure and keep the side of the lid farthest from you cracked just slightly, so a little of the steam can escape.