Because the basis can drift as the monitored entity's own state changes, such as a port's negotiated speed, this preserved value lets the admin recompute exactly what boundary was in force during the last incident rather than assuming the current basis was the same one that triggered it. For example, a basis of 100000000 preserved here, versus a current basis of 1000000000 after the port was upgraded, shows the last alert happened back when the link ran at only 100 Mbps.