They treat symptoms. There are no disease modifying medications for alzheimer's. Atypical or newer antipsychotic medications help with delusions, paranoia, suspicousness, fearfulness and the behaviors these symptoms cause. The older antipsychotics also help but have far more side effects. There is some evidence that antipsychotics increase the risk of stroke, heart attack or death but they do help quality of life.
Individualize care. The decision to use any medication for any symptom or illness involves a careful consideration of the risks and benefits for each individual. When someone has a progressive fatal illness, most people want to maximize quality of life and decrease stress and distress as much as possible. Antipsychotic medications are not a panacea and help only psychosis and aggressive symptoms, not other symptoms.