The lid prevents convective cooling, which is the cooling from air movement. Without the lid, the hot air at the surface of the water rises, taking some of the heat with it, which is then replaced with cooler air from the room. So you have to keep heating the air over and over when the lid is not on.
only_more_so t1_jdvcj8f wrote
Reply to ELI5: Why does water boil faster if it is in a covered pot? by AerisLionheart
The lid prevents convective cooling, which is the cooling from air movement. Without the lid, the hot air at the surface of the water rises, taking some of the heat with it, which is then replaced with cooler air from the room. So you have to keep heating the air over and over when the lid is not on.