" The rain tray is installed under the rear deck lid grill. Why do we need this?"
Very common for rain to get thru after market air cleaner into carbs - more so at the annual Carlisle Kit Car event. Someone must have cut one off of TRP's engine lid maybe to fit a large center carb, adapter and air cleaner. Many d plastic, FG or carbon fiber carb hats for further protection. Rain tray is better called a rain shield.