useMemo is for maintaining calculations of dependencies between renders, renders generally caused by state changes. React always tries to track state changes, but complicated states (represented by deep objects) can be recalculated too often - not sure if React 19 improves this, don't think so when reading documentation.

on edit: often useMemo is used by devs to cover up mistakes in rendering architecture. I believe React 19 improves stuff so you would not use useMemo, but not sure if it actually makes useMemo not at all useful.