Questions about free () function usage

The C Language Reference Manual describes the free () function as follows.

1) prototype of the free () function

Void free (void * ptr );

2) The free function destroys a memory area previously allocated by malloc, calloc, or realloc. The free function parameter must be the same pointer as the return value of a memory allocation function. If its parameter is a null pointer, calling this function will not produce any effect.

Since "if its parameter is a null pointer, calling this function will not produce any effect ." Why is it generally written as if (ptr! = NULL) free (ptr) instead of free (ptr? Should it be not just a good written programming style ?!

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