What is the purpose of the bottom of door threshold and how does it contribute to the overall functionality of a door?

The purpose of the bottom door threshold is to provide a seal at the bottom of the door to prevent drafts, moisture, and insects from entering a room. It also helps to maintain the temperature inside a room and contributes to energy efficiency by reducing air leakage. Additionally, the threshold provides a smooth transition between the floor surfaces on either side of the door, making it easier to open and close the door smoothly.