ISA MSL is 1013hPa.

Pressure altitue is vertical distance above 1013hPa pressure level, at about 1 hPa per 30 feet gain in height in the lower levels of the atmosphere.

We also have QNH and the local(aerodrome) QNH.

To determine the pressure altitude on the surface of an airfield which has an elevation of 1,875 ft if the QNH is 1,020 hPa.

Different between ISA MSL pressure and QNH = 1013 - 1020 = -7hPa

Different in pressure altitude = -7 x 30ft / hPa = -210 ft

Aerodrome elevation = 1,875

Therefore pressure altitude = 1875 - 210 = 1,665 ft.

Since we already know the altitude of aerodrome, why don't we just set the altimeter to 0 when we are on the ground of the aerodrome and if we fly within the local area.

When we need to fly to different areas, and above the certain altitude (e.g., 10,000 ft), then we just set the altimeter (subscale) to use the standard atmosphere which is 1013hPa.

I have read the page

https://www.brisbanehotairballooning.com.au/faqs/exam-help/133-pressure-density-height.html

even it does explain the reason of calculating it here "We can determine pressure height by either: reading the altimeter with 1013 set in the sub-scale; or by using the difference between QNH and 1013 to convert altitude to pressure height. If actual sea level pressure differs from the standard atmosphere of 1013 hPa, then a simple diagram will help us with any calculations of pressure height. We convert altitude to pressure height by allowing 30 feet for each 1 hPa pressure difference."

but when I look at the picture in EXAMPLE 1, and we got

Elevation = 670 feet

QNH = 1020 hPa

Pressure Height = 670 - 7 x 30 = 460 feet

Why we need to know the Pressure Height between MSL and the aerodrome level which is (460 ft), in that case, should I set the altimeter to 1020 QNH in the subscale?

I don't know how this useful to me.

[Edited 2012-09-06 07:30:52]