It’s just for progression, economic, and task reasons. Namely Tech tree progression as you in most cases need a certain number of vehicles in a specific rank to unlock the next. Generally 6 or 5 vehicles is the required number of vehicles to unlock the next rank. A example is German ground rank 5. You need 6 rank 4 german tanks to unlock rank 5 german tech tree vehicles. Another more special example is helicopters as in order to unlock the helicopter tech tree in a certain nation you need to reach a tank or plane at rank 5 or higher in that nation. Premium, Squadron, and event vehicles will count towards both examples alongside normal tech tree ones. Also all special tasks require you to play rank 3 or higher to complete