Eggplant naturally has a lot of water in it. Salting and allowing it to sit and drain for a while before cooking will draw out some of the water. Additionally, it will remove some of the natural bitterness some eggplant may have. This dish can come out pretty watery, so salting in advance helps to cut that down a bit . Both the salt and the any excess fluid it draws out should be wiped off of your eggplant before cooking.