I would like to know the purpose of portrait orientation lock

Portrait orientation keeps your iPhone for rotating the screen when the device is physically rotated.

So the screen remains locked to the same orientation regardless of how the iPhone is held or positioned.

With portrait orientation lock turned off, the screen can rotate, and always be right side up regardless of how the phone is physically oriented.

iOS 11 User Manual - Orientation Lock - Apple Support

It prevents the phone from going into landscape mode when you turn the device.