If you do not have a backwater valve in your basement plumbing you are subject to all the problems the city may have as a backup can happen at any time. The backwater valve is the only protection between you and the city sewer. When the city has high volumes of water such as after a downpour it can sometimes have a hard time keeping up with the flow of water coming at the treatment facility. Often the sewer lines are old and too small to handle high volumes so the water will rise up into your basement drain. If you have the back water valve and it is working properly it can hold back the water. Without the valve the water will usually flow into the basement through the floor drain which is the lowest point of entry. From my experience it is usually about $10-$30,000 in damage to a finished basement when only a couple inches of water enters the basement. A backwater valve is essential to prevent this.