I’ll admit that the arched-back bench press looks strange.  Kind of like a back injury waiting to happen.  However, when done correctly,  it’s safe and is the preferred technique used for powerlifting.  

Why?  Because the arched position shortens the range of motion needed in competition to push a heavy barbell upward.  

Is it necessary?  Depends on your training goals.   If your trying to build general strength or training for another sport, the flat-back technique is perfectly fine to use. 

Check out the comparison below or this article from the Strength & Conditioning Journal.