Explain why a retrospective delusion reshaping personal history "to fit" a current delusion is considered clinically useful information, rather than simply an incidental distortion.

The reshaping of personal history to fit a current delusion is itself a useful marker of how deeply the delusion has been integrated into the person's overall self-understanding, rather than remaining a discrete, isolated belief held alongside an otherwise intact sense of their own history. When a delusion has been extended backward across a whole life narrative, this suggests a more entrenched, pervasive disturbance than a delusion that is confined to recent events. Checking a patient's current account against documented or collateral history can reveal this backdating even when the patient presents their reshaped history with complete conviction, making it a useful, low-burden way of gauging how fully a delusional system has come to organise the person's sense of who they are.