Precision
BaZi's hour pillar comes from the position of the sun, not from your wall clock. Two people born at the same clock moment in different cities can belong to different two-hour periods (shichen), because solar noon arrives at different clock times along a meridian.
The conversion runs: clock time → legal standard time (removing daylight saving — China used DST in 1986–1991, and many countries still do) → true solar time, by adding the longitude correction (four minutes per degree from the time zone's standard meridian) and the equation of time, the astronomical wobble between sun and clock that swings roughly ±15 minutes across the year.
For most births the correction moves the chart little or not at all. But hours have edges — a birth at 09:00 by the clock in far-western China, with DST in force, can convert to a true solar reading of 07:50 and land in a different shichen. Edge cases are exactly where precision pays.
This is also why a calculator should ask for a real place, not just a country: the place fixes the longitude and the historical time zone, including daylight saving rules as they applied in that year. Good charts show both readings — the clock time and the solar time — so the basis is always visible.