Behavioural and Psychological Symptoms of Dementia (BPSD) are of various types. Some patients may have psychotic symptoms (suspiciousness that people are harming them or cheating them of their money etc.) or hear imaginary voices - hallucinations. Antipsychotic medications such as risperidone, quetiapine or haloperidol are used for such symptoms.
Aggression, wandering aimlessly can also be seen in the absence of psychotic symptoms. In such cases too, antipsychotics can help control symptoms. However, antipsychotics are to be used in minimal amounts in dementia - minimise the duration and dose as far as possible because they can cause increased health problems in these patients.

Hence, valproic acid/ divalproex is sometimes used in patients who are aggressive but do not have psychotic symptoms. It is probably a safer option with lesser side-effects than long term antipsychotics in patients with dementia. Otherwise you are right that valproic acid is mainly used for bipolar disorder and epilepsy.