Sometimes when roasting a pork loin, you don’t get a lot of drippings (juices that drip out of the roast as it cooks). Instead, what sometimes happens is you get just a little bit of drippings and those drippings hit the hot roasting pan and then they can burn onto there. If you were later going to deglaze that pan to make gravy, there would be a bitter burned flavor from the drippings. A little bit of broth or stock in the pan gives the drippings somewhere safe to fall. They’ll drip into the broth/stock and then won’t burn. The flavor that broth gives makes it better as juices for gravy later. Also, as the broth evaporates, it leaves some brownings on the sides of the roasting pan. Make sure you either scrape those into your gravy juices, or pour more broth in to dissolve that flavor. So, if you’re making gravy, then put some broth or stock in the bottom of your roasting pan. Half an inch will do. If you’re not making gravy, then the broth isn't necessary.