`#pragma once` is to prevent a header from being reparsed repeatedly for the same translation unit when ten different headers all include a common one transitively.

it replaces the prior pattern of including code predicated on a unique definition defined only within that same block to avoid double parsing.

    /* if it isn't unique, you're going to have a bad time */
    #ifndef SOME_HOPEFULLY_UNIQUE_DEFINITION
    #define SOME_HOPEFULLY_UNIQUE_DEFINITION
    
    ...code...
    
    #endif /* SOME_HOPEFULLY_UNIQUE_DEFINITION */

This, however, doesn't stop those same headers from needing to be reread and reparsed and reread and reparsed for every single cpp file in the project.