Sometimes people are brought in because they have committed an offence and cannot be kept in custody or in prison and we need to find out if they are fit to plead in court. Sometimes they have become unwell in the community and get sectioned for their own safety. Sometimes they come from other hospitals... most commonly they're in the mania phase of a schizophrenic episode, are exhibiting full-blown psychosis, or are self-harming to such an extent they cannot be managed by lower security services.