A service-principal sign-in log records authentication activity by an application identity, rather than by a human user. In Microsoft Entra, it can help show when an application authenticated, which identity it used, and whether the sign-in succeeded or failed.
These records matter because applications may have permissions to access cloud resources without a user being present. Defenders review service-principal sign-ins to spot unfamiliar activity, investigate exposed credentials, and understand when an application authenticated. A successful sign-in confirms authentication, not that data was accessed or stolen; investigators should compare the log with audit and resource activity records. Reviewing app permissions and limiting them to what the application needs can reduce risk.