When you’re rebuilding an automotive head The bottom of the head Hass to be flat. If not the head gasket will not affectively seal. You could take it to a machine shop but then you have to wait for it and it cost a bit more. With two dollars of sandpaper and a good piece of glass you can machine this yourself. We’ve been doing this for years and it works really well. If the head is not warped it’s just nice to do this to clean it up a bit to verify that everything is in good shape