When it is in the normal execution path, the test avoids a function call, so there's a little bit of performance gain. It can also be a bit self-documenting, o

When it is in the normal execution path, the test avoids a function call, so there's a little bit of performance gain. It can also be a bit self-documenting, or be a place-holder if there is for example stuff to delete in a loop (elements of an array) before releasing the pointer itself (the array). I don't know, when you read it, sometimes it feels natural and meaningful, and sometimes you know that the author didn't know that free() handles NULL all right.