With OEI if an aircraft is trimmed for zero side slip it will have some drag due to side loading. The minimum drag position is some slip neutralised by some bank. But how much?Airbus does the calculation for the pilot by creating the beta target which leaves some slip. Pilot just trims the rudder and flies the heading with side stick. This is the minimum drag for optimum OEI performance. Yes as John pointed out there is no bank without aileron but still it is the least drag position.