By pointing backwards, it minimizes the turbulence going into the carb. Facing forward, for example, would cause pressure changes if it was exposed to in-rushing air. The tube connects the reference (or static port) of the diaphragm regulator inside the half of the carb that provides the regulation of fuel pressure to the needle valves. Without this, the cowl pressure changes can push or pull the regulator diaphragm affecting the pressure that goes to the needle valves. There is no fuel or air passing through this tube. Just minor amounts of air as the diaphragm pulses in and out.