Aluminium foil can be used as a substitute. If you use a wooden lid, it is best to wet it before use to avoid the smell from the pan being absorbed by the lid. It should not be dried in direct sunlight, as this will warp it. The lid prevents the ingredients from breaking up too much during simmering. It also stops the flavour in the dish from evaporating and allows the stock to be spread more evenly throughout the pan. There are simmered dishes that need a drop lid and others that don’t, but if in doubt, it is best to err on the side of caution and use one.