The directive provides a single, well-defined "escape hatch" which can be used for
all sorts of (nonportable) implementation-specific controls and extensions: source
listing control, structure packing, warning suppression (like lint's old /* NOTREACHED
*/ comments), etc. 

For example


#pragma once


inside a header file is an extension implemented by some preprocessors to help make
header files idempotent (to prevent a header file from included twice).