An exact time changes the Ascendant, Midheaven, houses, angularity, Human Design details, astrocartography lines, and relocation structure. It can also matter in relationship overlays and timing techniques. The catalogue labels time as required, recommended, or unnecessary for each mode instead of making one blanket statement.
When time is unknown, date-based or current-sky work may still be useful. The responsible approach is to hide or qualify details that depend on a precise time rather than silently substituting certainty.