In the Anomaly settings of a cloud security platform, choosing a High alert disposition severity is crucial when the objective is to generate alerts for both low and high severity incidents, especially in scenarios such as reporting unknown browsers and OS, impossible time travel, or account hijacking attempts. Setting the alert severity to High ensures that the security team is promptly notified of both overt and subtle anomalies, enabling quick response to potential security threats that may indicate unauthorized access or suspicious activities within the cloud environment.