NULL is used to make it clear it is a pointer type.

Ideally, the C implementation would define NULL as ((void *) 0) or something equivalent, and programmers would always use NULL when they want a null pointer constant.

If this is done, then, when a programmer has, for example, an int *x and accidentally writes *x = NULL;, then the compiler can recognize that a mistake has been made, because the left side of = has type int, and the right side has type void *, and this is not a proper combination for assignment.

In contrast, if the programmer accidentally writes *x = 0; instead of x = 0;, then the compiler cannot recognize this mistake, because the left side has type int, and the right side has type int, and that is a valid combination.

Thus, when NULL is defined well and is used, mistakes are detected earlier.

In particular answer to your question “Is there a context in which just plain literal 0 would not work exactly the same?”:

• In correct code, NULL and 0 may be used interchangeably as null pointer constants.
• 0 will function as an integer (non-pointer) constant, but NULL might not, depending on how the C implementation defines it.
• For the purpose of detecting errors, NULL and 0 do not work exactly the same; using NULL with a good definition serves to help detect some mistakes that using 0 does not.

The C standard allows 0 to be used for null pointer constants for historic reasons. However, this is not beneficial except for allowing previously written code to compile in compilers using current C standards. New code should avoid using 0 as a null pointer constant.