The angled spacer creates a standoff for the rear O2 sensor so that it only sees a small portion of the real exhaust flow. It is basically a hardware mod that makes the rear O2 sensor/ECU think it is seeing the same exhaust flow with a high flow cat installed that it saw previously with a standard low flow cat.

On a standard car with a high flow exhaust and no angled spacer, the rear o2 sensor will basically see huge exhaust flow compared to what the ECU expects to see. ie. out of range readings. The ECU will assume an exhaust malfunction and trigger the warning light.

The spacer is needed if you have a standard ECU or a tune that hasn't had the exhaust warning ticked off.

A tune written to prevent the CEL is guaranteed to work and may also have more appropriate fueling/timing to go with the exhaust. Using a spacer is is less certain to prevent the CEL.