However, the main purpose of manhole cover why it is preferable to make it in round shape is to prevent it from falling into the circular opening. All manholes, whether it's a sewer or not, it will be created with a lip around the rim of the hole, with a size small enough than the manhole cover dimensions. Unlike if it is in rectangular, square, or oval shape, even if the hole is smaller and the cover is a bit larger, once the manhole cover will be inserted into the hole diagonally then there is a possibility that it could fit in and fall. With the circular shape, inserting diagonally still won't work to fit it and slip into the hole which make it more safe.