The bifold door sill is often used to raise the bifold door assembly in door openings that are higher than a normal opening. The sill can also be used to secure bifold doors that are separating a room from the outside. If you are putting in a bifold door that will be exposed to the outdoors, then a sill is necessary to create a weather seal and protect the floor inside the house from the elements. If you need a half-inch of extra height on your interior bifold doors, then a sill will make it look professional. Otherwise, a sill would not be necessary.