A roasting pan should have a lid. A covered roasting pan will cook food quicker than an uncovered one. It creates a mini-oven within your regular oven.
The lid traps moisture which condenses and then drips back on the food. You can remove the lid if you want the meal to have a crispy crust. In case browning is happening too fast before the meat is cooked to the right temperature, the roasting pan lid will slow the process. The lid may have corrugated or protruding areas to encourage the condensation and dripping.
A lid reflects heat back onto the food ensuring more even cooking. The lid can also help food stay warm after you remove it from the oven. Some roasting pan lids can be flipped over and used as a roasting pan in their own right. You have more cooking options when your roasting pan has a lid than when it does not.