The only way station pressure doesn't need any adjustment is if the "station" is at the exact altitude you're shooting at and in the local vicinity. Since National Weather Service (NWS) reporting stations across the country are located at numerous altitudes, their reports are normalized to sea level to allow the NWS to make maps of pressure changes over distance. The normalized sea level pressure is also useful for anyone nearby by making a simple adjustment using the altitude at your current location.
